MOSCOW — Russia's foreign minister said Saturday that the evidence put forth by the United States of chemical weapons use in Syria apparently doesn't meet stringent criteria for reliability.

The Obama administration said this week that it will give lethal aid to Syrian rebels in light of evidence that President Bashar Assad's forces used chemical weapons in the country's civil war.

In Moscow, Foreign Minister Sergey Lavrov said the material does not include guarantees that it meets the requirements of the Organization for the Prohibition of Chemical Weapons. He said the organization specifies that samples taken from blood, urine and clothing can be considered reliable evidence only if supervised by organization experts from the time they are taken up to delivery to a laboratory.

The OPCW is the autonomous body for implementing the international Chemical Weapons Convention that went into effect in 1997. Its website says Syria is one of six countries that have not signed or acceded to the convention.

A spokesman for the organization, based in The Hague, Netherlands, could not immediately be reached for comment.

Lavrov, after meeting with his Italian counterpart Emma Bonino, scoffed at suggestions that Assad's regime would use chemical weapons now in light of its apparent growing advantage against the rebels.

"The regime doesn't have its back to the wall. What would be the sense of the regime using chemical weapons, moreover at such a small quantity?" he said.

Russia has blocked proposed U.N. sanctions against Assad's regime and acknowledged last month that it has contracted to supply advanced S-300 air-defense missiles to Syria. But President Vladimir Putin and other officials say the policies do not constitute overt support for Assad.

Also on HuffPost:

Loading Slideshow...
  • A rebel fighter of the Syrian Kurdish Popular Protection Units (YPG) has a flower in his Russian made 'AK-47' kalashnikov gun as he holds a position in the Sheikh Maqsoud neighbourhood in the Syrian city of Aleppo on May 9, 2013. (STR/AFP/Getty Images)

  • In this Sunday, May 5, 2013 file photo provided by the Syrian official news agency SANA, workers clear the rubble of buildings destroyed by an Israeli airstrike in Damascus, Syria. The attack targeted a shipment of highly accurate, Iranian-made guided missiles believed to be on their way to Lebanon's Hezbollah militant group, officials and activists said. (AP Photo/SANA, File)

  • In this Sunday, May 5, 2013 image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, armed men stand near the wreckage of a military helicopter, left, in Deir el-Zour, Syria. Syrian rebels shot down a military helicopter in the country's east, killing eight government troops on board a day after opposition forces entered a sprawling military air base in the north, activists said Monday. (AP Photo/Shaam News Network via AP video)

  • A view of the 13th medieval century citadel that dominates the city of Aleppo, in northern Syria, seen through a sniper position at the 'Izaa' radio frontline, on May 11, 2013. (STR/AFP/Getty Images)

  • Syrian National Defence Forces (NDF), an armed unit of volunteers loyal to President Bashar al-Assad operating under Syrian army command, patrol the village of Ghassaniyeh in the countryside of the rebel city of Qusayr on May 11, 2013. (JOSEPH EID/AFP/Getty Images)

  • This image taken from video obtained from Ugarit News, which has been authenticated based on its contents and other AP reporting, shows a rocket fired by Syrian rebels at Mannagh air base in Aleppo province, Syria, Monday, May 13, 2013. (AP Photo/Ugarit News via AP video)

  • This citizen journalism image provided by Lens Young Homsi, which has been authenticated based on its contents and other AP reporting, shows buildings which were destroyed from Syrian forces shelling, in Homs province, Syria, Monday, May 13, 2013. (AP Photo/Lens Young Homsi)

  • This Tuesday, May 14, 2013 citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows the mother of a Syrian rebel cleaning a rifle, in Aleppo, Syria. (AP Photo/Aleppo Media Center AMC)

  • This photo released by the Syrian official news agency SANA, claims to show a Syrian army soldier inspecting a damaged car at the scene of a car bomb which exploded, in Damascus, Syria, Saturday, May 18, 2013. A car bomb killed at least three people and wounded five, according to Syrian state TV. It said bomb experts dismantled other explosives in the area. (AP Photo/SANA)

  • This citizen journalism image provided by Qusair Lens which has been authenticated based on its contents and other AP reporting, shows Syrian rebels preparing to repel a coordinated attack by government forces, in Qusair, Homs province, Syria, Sunday, May 19, 2013. (AP Photo/Qusair Lens)

  • This citizen journalism image provided by Qusair Lens which has been authenticated based on its contents and other AP reporting, shows Syrian rebels gathering at one of the front lines, in Qusair, Homs province, Syria, Monday May 20, 2013. (AP Photo/Qusair Lens)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, smoke and fire rises after explosives were dropped by a Syrian government warplane in Yabroud near Damascus, Syria, Monday May 20, 2013. (AP Photo/Shaam News Network via AP video)

  • In this image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, smoke rises after explosives were dropped by a Syrian government warplane in Yabroud near Damascus, Syria, Monday May 20, 2013. (AP Photo/Shaam News Network via AP video)

  • Lebanese mourners carry the coffin of Hezbollah fighter Hassan Faisal Shuker, 18, who killed in battles against the Syrian rebels at the Syrian town of Qusair, during his funeral procession at his hometown of Nabi Sheet in the eastern Bekaa valley, Lebanon, Monday May 20, 2013. (AP Photo)

  • Citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, show buildings damaged during battles between the rebels and the Syrian government forces, in the Salaheddine neighborhood of Aleppo, Syria, Monday May 20, 2013. (AP Photo/Aleppo Media Center, AMC)

  • Citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, show flames rising from a car which was attacked by Syrian government forces, in Bustan al-Qaser neighborhood, Aleppo, Syria, Monday May 20, 2013. (AP Photo/Aleppo Media Center, AMC)

  • This Tuesday, May 21, 2013 file citizen journalism image provided by Qusair Lens, which has been authenticated based on its contents and other AP reporting, shows Syrian citizens gathering over houses that were destroyes from a Syrian forces air strike in the town of Qusair, near the Lebanon border, Homs province, Syria. (AP Photo/Qusair Lens, File)

  • This Tuesday, May 21, 2013 file citizen journalism image provided by Qusair Lens, which has been authenticated based on its contents and other AP reporting, shows Syrian citizens inspecting the rubble of damaged buildings that were damaged from a Syrian forces air strike in the town of Qusair, near the Lebanon border, Homs province, Syria. (AP Photo/Qusair Lens, File)

  • This Tuesday, May 21, 2013 citizen journalism image provided by Edlib News Network, ENN, which has been authenticated based on its contents and other AP reporting, shows Syrian rebels firing locally made shells made from gas cylinders against the Syrian forces, in Idlib province, northern Syria. Syria's main opposition group is urging rebels to come from around the country to reinforce Qusair, a western town under attack by Syrian troops and members of Lebanon's Hezbollah group. (AP Photo/Edlib News Network ENN)

  • This citizen journalism image provided by Qusair Lens, which has been authenticated based on its contents and other AP reporting, shows a Syrian man checking his destroyed house that was damaged by a Syrian forces air strike in the town of Qusair, near the Lebanon border, Homs province, Syria, Tuesday, May 21, 2013. (AP Photo/Qusair Lens)

  • This citizen journalism image provided by Qusair Lens, which has been authenticated based on its contents and other AP reporting, shows Syrian men carrying copies of the Muslim holy book, the Quran, inside a bombed mosque in the town of Qusair near the Lebanon border, Homs province, Syria, Tuesday, May 21, 2013. (AP Photo/Qusair Lens)

  • Lebanese mourners carry the coffin of Hezbollah fighter Saleh Sabagh, 18, who was killed in a battle against Syrian rebels in the Syrian town of Qusair, during his funeral procession, in the southern port city of Sidon, Lebanon, Wednesday, May 22, 2013. Dozens of supporters of hardline Sunni Muslim cleric Ahmad al-Assir on Wednesday blocked the road leading to a cemetery in the southern city of Sidon to prevent the burial of a Hezbollah fighter who died recently in Qusair. (AP Photo/Mohammed Zaatari)

  • This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows Syrian rebels standing in the middle of a medieval market in Old Aleppo, which has been destroyed by fighting between regime forces and rebels seeking to topple Syrian President Bashar Assad, in Aleppo, Syria, Thursday, May 23, 2013. (AP Photo/Aleppo Media Center AMC)

  • In this photo released on Sunday, May 26, 2013, by the Syrian official news agency SANA, Syrian forces loyal to Syrian President Bashar Assad take their position during a clashes against Syrian rebels, in Aleppo, Syria. (AP Photo/SANA)

  • In this photo released on Sunday May 26, 2013, by the Syrian official news agency SANA, Syrian forces loyal to Syrian President Bashar Assad run to take their position during clashes against Syrian rebels, in Aleppo, Syria. Syria's Information Ministry says rebels have killed a TV correspondent who was covering clashes near the border with Lebanon. (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, Syrians participate in the funeral prayer for Youssef Ghazi al-Sarmani who was killed in fighting between rebel and government forces, May 27. Logo in red reads, "Talbiseh". (AP Photo/Shaam News Network via AP video)

  • In this Monday, May 27, 2013 photo provided by Mouaz Moustafa and the Syrian Emergency Task Force, Sen. John McCain, R-Ariz., center, accompanied by Moustafa, right, visits rebels in Syria. McCain, who slipped into the country for a surprise visit, favors providing arms to rebel forces in Syria. (AP Photo/Syrian Emergency Task Force, Mouaz Moustafa)

  • This image made from video posted by Shaam News Network and taken on Wednesday, May 29, 2013, which is consistent with other AP reporting, shows an explosion from shelling in Rastan, Syria. (AP Photo/Shaam News Network via AP video)

  • This image from amateur video obtained by a group which calls itself Ugarit News, which is consistent with AP reporting, shows a rocket fired by Syrian rebels in Qusair, Syria, Tuesday, May 28, 2013. (AP Photo/Ugarit News via AP video)

  • This photo released by the Syrian official news agency SANA, shows Syrian forces loyal to Syrian President Bashar Assad at the Dabaa military air base, in Homs province, Syria, Thursday, May 30, 2013. The Britain-based Syrian Observatory for Human Rights reported ongoing clashes in the town on Thursday. The Observatory called for urgent aid to the injured inside the town, most of which is now controlled by Assad’s troops, including the Dabaa military air base just outside Qusair. (AP Photo/SANA)

  • This citizen journalism image provided by Qusair Lens, which has been authenticated based on its contents and other AP reporting, shows Qusair-based activist Hadi Abdullah, right, walking on a street hit by the shelling of Hezbolllah Lebanese Shiite group and the Syrian forces loyal to Syrian President Bashar Assad, in the town of Qusair, near the Lebanon border, Homs province, Syria, Friday, May 31, 2013. (AP Photo/Qusair Lens)