GENEVA — Evidence suggests that some kind of "substance" was used in Syria that may have killed more than 1,000 people, but any military strike in response must first gain U.N. Security Council approval, special envoy to Syria Lakhdar Brahimi said Wednesday.

Brahimi spoke to reporters in Geneva as U.N. chemical weapons experts headed to a Damascus suburb for another look at the alleged poison gas attack near Damascus on Aug. 21 and as momentum built for Western military action against Syrian President Bashar Assad's regime.

"With what has happened on the 21st of August last week, it does seem that some kind of substance was used that killed a lot of people: hundreds, definitely more than a hundred, some people say 300, some people say 600, maybe 1,000, maybe more than 1,000 people," said Brahimi, who has been the U.N. and Arab League's special envoy to Syria since August 2012.

"This was of course unacceptable. This is outrageous. This confirms how dangerous the situation in Syria is and how important for the Syrians and the international community to really develop the political will to address this issue seriously, and look for a solution for it," he said.

Brahimi did not say on what he based his information, but he did discuss the work of the U.N. team probing for evidence of the use of chemical weapons.

"The United Nations has inspectors on the spot. They have already spent one day in one area where this substance – whatever it is – has been used," he said. "They have come back with a lot of samplings, they talked also to doctors, they talked to witnesses."

He said his information did not come from Western intelligence, including what U.S. Secretary of State John Kerry has called "undeniable" evidence of a large-scale chemical attack likely launched by Assad's regime.

"What we have been told is that this evidence that the Americans, the British, the French say they have is going to be shared with us. It hasn't been until now," he said. "And we will be very, very, very interested in hearing from them what this evidence they have is."

Syria has one of the world's largest stockpiles of chemical charges, but denies the charges. U.S. leaders have not presented their proof, and U.N. inspectors have yet to comment on the allegations.

Brahimi called the civil war – which has killed more than 100,000, forced about 2 million refugees from their homes and displaced millions of others – the most serious crisis facing the international community.

But he said that any U.S.-led military action must first gain approval from the 15-nation Security Council, whose five permanent members – Britain, China, France, Russia and the United States – each have veto power. The United States has been laying the groundwork for a possible strike on Syria, but U.N. Secretary-General Ban Ki-moon has urged nations to allow more time for diplomacy.

Britain says it will seek a measure "authorizing necessary measures to protect civilians" in Syria. They want it drafted under Chapter 7 of the U.N. Charter, which is militarily enforceable, when it is a political resolution that doesn't mandate any specific actions.

"International law says that military action must be taken after a decision by the Security Council," Brahimi said. "I must say that I do know that President Obama and the American administration are not known to be trigger-happy.-What they will decide, I don't know. But certainly international law is very clear: the Security Council has to be brought in."

Military intervention would certainly complicate Brahimi's efforts toward hosting a second Geneva peace conference to find a political settlement in Syria, as a follow-up on a June 2012 foreign ministers' conference, which isn't likely to occur until October or later. Brahimi moved his base of operations to Geneva this month for that reason, but he said Wednesday that he merely hopes a conference can be pulled off by the end of the year.

Although he is "against military intervention by principle," Brahimi said decisive action must come soon.

"If they are responsible for what has happened on the 21st of August," he said of the Assad regime, "I don't think you will find many people who think that this is the first outrageous thing that has been done by this regime.

"One hundred thousand people have been killed. Quite a few of them have been killed by the regime. So I think, from my point of view, if there is one complaint about the international community, it is that they haven't done enough before the 21st August. If they can do more after the 21st August, then that would be great. "

This is a developing story. More to come...

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  • Syrian refugee, Ahmed al Delly, 59, from Daraa in Syria, reacts as he speaks about his wife, four sons, and two daughters, who are still in Daraa but he has had no contact with them, after the prayer of Eid al-Fitr, that marks the end of the holy fasting month of Ramadan, at Zaatari Syrian refugee camp, in Mafraq, Jordan, Thursday, Aug. 8, 2013. (AP photo/Mohammad Hannon)

  • This image provided by Shaam News Network on Thursday, Aug. 22, 2013, which has been authenticated based on its contents and other AP reporting, purports to show mourners next to bodies of victims of an attack on Ghouta, Syria on Wednesday, Aug. 21, 2013. (AP Photo/Shaam News Network)

  • This image provided by by Shaam News Network on Thursday, Aug. 22, 2013, which has been authenticated based on its contents and other AP reporting, purports to show several bodies being buried in a suburb of Damascus, Syria during a funeral on Wednesday, Aug. 21, 2013. (AP Photo/Shaam News Network)

  • Columns of smoke rising from heavy shelling in the Jobar neighborhood in west Damascus, in Cairo, Syria, Thursday, Aug. 22, 2013. (AP Photo/Hassan Ammar)

  • In this citizen journalism image provided by the Media Office Of Douma City, which has been authenticated based on its contents and other AP reporting, a Syrian man mourns over a dead body after an alleged poisonous gas attack fired by regime forces, according to activists, in Douma town, Damascus, Syria on Wednesday, Aug. 21, 2013. (AP Photo/Media Office Of Douma City)

  • Syrian refugees cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • This citizen journalism image provided by the Local Committee of Arbeen which has been authenticated based on its contents and other AP reporting, shows a Syrian man reacts as he carries a dead body of a Syrian girl, after an alleged poisonous gas attack fired by regime forces, according to activists in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Local Committee of Arbeen)

  • This citizen journalism image provided by the Local Committee of Arbeen which has been authenticated based on its contents and other AP reporting, shows Syrian citizens trying to identify dead bodies, after an alleged poisonous gas attack fired by regime forces, according to activists in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Local Committee of Arbeen)

  • This citizen journalism image provided by the Local Committee of Arbeen which has been authenticated based on its contents and other AP reporting, shows dead bodies of Syrian citizens in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Local Committee of Arbeen)

  • A Syrian family sits in a tent at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees stand in a queue to receive free food at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees wait to receive a tent at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees wait to receive a tent at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees gather for food aid at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees wait for food aid at Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • A Syrian military soldier holds his AK-47 with a sticker of Syrian President Bashar Assad as he stands guard at a check point on Baghdad street, in Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Hassan Ammar)

  • A Syrian military soldier holds his AK-47 with a sticker of Syrian President Bashar Assad and Arabic that reads, "Syria is fine," as he stands guard at a check point on Baghdad street, in Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Hassan Ammar)

  • This citizen journalism image provided by the Local Committee of Arbeen which has been authenticated based on its contents and other AP reporting, shows a Syrian man receiving treatment, in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. (AP Photo/Local Committee of Arbeen)

  • Syrian refugees cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • Syrian refugees cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • A general view of the Kawergost refugee camp in Irbil, 217 miles (350 kilometers) north of Baghdad, Iraq, Wednesday, Aug. 21, 2013. (AP Photo/Hadi Mizban)

  • In this image taken from video posted by Ugarit News, which has been authenticated based on its contents and other AP reporting, purports to show a fireball from an explosion at a weapons depot set off by rocket attacks that struck government-held districts in the central Syrian city of Homs on Thursday, Aug. 1, 2013. (AP Photo/Ugarit News via AP video)

  • Syrian refugees gather for food aid after cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, a Syrian man comforts a child injured by a missile strike in Raqqa, Syria, Wednesday, Aug. 7, 2013. (AP Photo/Shaam News Network via AP video)

  • Syrian refugees cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • In this citizen journalism Monday, Aug. 12, 2013, image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, buildings are seen damaged from the shelling of Syrian forces at Karm al-Jabal area in Aleppo province, Syria. (AP Photo/Aleppo Media Center, AMC)

  • In this citizen journalism Monday, Aug. 12, 2013, image provided by Aleppo Media Center AMC, which has been authenticated based on its contents and other AP reporting, Syrian rebels open fire as they battle against the Syrian forces in Aleppo, Syria. (AP Photo/Aleppo Media Center, AMC)

  • In this image released by the Syrian official news agency SANA, a Syrian army soldier stands behind his machine gun during a battle against the Syrian rebels at an unidentified location in Latakia province, Syria, Tuesday, Aug. 13, 2013. (AP Photo/SANA)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, a Syrian man carries an injured child away from a missile strike in Raqqa, Syria, Wednesday, Aug. 7, 2013. (AP Photo/Shaam News Network via AP video)

  • In this image released by the Syrian official news agency SANA, a Syrian forces tank fires during a battle against the Syrian rebels at an unidentified location in Latakia province, Syria, Tuesday, Aug. 13, 2013. (AP Photo/SANA)

  • In this image taken from Sunday, Aug. 11, 2013, video obtained from the Sham News Network, which has been authenticated based on its contents and other AP reporting, a rebel fighter fires a gun in a valley in an unidentified location in Latakia province, Syria. (AP Photo/Sham News Network via AP video)

  • Mansour Mahameed, 26, from Daraa city in Syria, a former Free Syrian Army fighter who lost his leg last March after the Syrian troops bombing, prepares to sit for the Eid al-Fitr prayer that marks the end of the holy fasting month of Ramadan at Zaatari Syrian refugee camp in Mafraq, Jordan, Thursday, Aug. 8, 2013. (AP Photo/Mohammad Hannon)

  • Syrian refugees cross into Iraq at the Peshkhabour border point in Dahuk, 260 miles (430 kilometers) northwest of Baghdad, Iraq, Tuesday, Aug. 20, 2013. (AP Photo/Hadi Mizban)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, a Syrian man carries an injured child away from a missile strike in Raqqa, Syria, Wednesday, Aug. 7, 2013. (AP Photo/Shaam News Network via AP video)

  • In this photo released by the Syrian official news agency SANA, dead bodies of Syrian rebels are seen on the ground, killed during ambush by Syrian forces near the Damascus suburb of Adra, Syria, Wednesday Aug. 7, 2013. (AP Photo/SANA)

  • In this image taken from video posted by Shaam News Network, which has been authenticated based on its contents and other AP reporting, purports to show a fireball from an explosion at a weapons depot set off by rocket attacks that struck government-held districts in the central Syrian city of Homs on Thursday, Aug. 1, 2013. (AP Photo/Shaam News Network via AP video)

  • This image posted on the official Facebook page of the Syrian Presidency on Thursday, Aug. 1, 2013 purports to show Syrian President Bashar Assad shaking hands with a soldier during Syrian Arab Army day in Darya, Syria. (AP Photo/Syrian Presidency via Facebook)

  • This image posted on the official Facebook page of the Syrian Presidency on Thursday, Aug. 1, 2013 purports to show Syrian President Bashar Assad walking with soldiers with during Syrian Arab Army day in Darya, Syria. (AP Photo/Syrian Presidency via Facebook)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, a Syrian youths run away from the scene of a missile strike in Raqqa, Syria, Wednesday, Aug. 7, 2013. (AP Photo/Shaam News Network via AP video)