JERUSALEM -- President Barack Obama said Wednesday that the United States is investigating whether chemical weapons have been deployed in Syria, but he's "deeply skeptical" of claims by Syrian President Bashar Assad's regime that rebel forces were behind such an attack.

Both the Assad regime and Syrian rebels have accused each other of using chemical weapons in an attack on Tuesday that the government says killed 31 and wounded more than 100. But Obama suggested it's more likely that if the weapons were used, the Syrian government was behind the attack.

"We know the Syrian government has the capacity to carry out chemical weapon attacks," Obama said. "We know that there are those are in the Syrian government who have expressed a willingness to use chemical weapons if necessary to protect themselves. I am deeply skeptical of any claim that in fact it was the opposition that used chemical weapons. Everybody who knows the facts of the chemical weapons stockpiles inside of Syria as well as the Syrian government capabilities, I think, would question those claims."

"Once we establish the facts, I have made clear that the use of chemical weapons is a game changer," Obama said in a news conference with Israeli Prime Minister Benjamin Netanyahu. Obama said he wouldn't announce what the next steps would be while the investigation is unfolding. But he echoed his statement over the summer that the use of chemical weapons in Syria would be a "red line" for the United States.

"When you start seeing weapons that can cause potential devastation and mass casualties and you let that genie out of the bottle, then you are looking potentially at even more horrific scenes than we've already seen in Syria. And the international community has to act on that additional information," Obama said.

"We have been clear that the use of chemical weapons against the Syrian people would be a serious and tragic mistake," Obama said.

Obama said the U.S. policy not to intervene militarily thus far is based on his desire to solve the problem as a global community. "It's a world problem ... when tens of thousands of people are being slaughtered, including innocent women and children," Obama said.

Netanyahu said the two leaders discussed Syria during their private meeting earlier. He said the two countries share a goal of preventing Syria's weapons arsenal from falling into the hands of terrorists.

Obama said the United States shares the concern that the weapons could be transferred to a group like Hezbollah and used against Israel. "The Assad regime must understand that they will be held accountable for the use of chemical weapons or their transfer to terrorists," Obama said.

The president's first comments on the reports came shortly after the U.S. ambassador to Syria, Robert Ford, told Congress of an untenable situation in Syria as the civil war grinds into its third year. The United Nations has estimated 70,000 have been killed, more than 1 million refugees have fled to neighboring countries and 2.5 million have been displaced internally.

The Syrian people "face a new level of ruthlessness from the Assad regime, which is raining Scud missiles down on residential neighborhoods, destroying hospitals and schools, and sending its thugs rampaging through the streets to terrorize their fellow citizens. The carnage is appalling," Ford said.

He insisted that the ideal outcome is a "negotiated political transition" to the crisis without Assad.

Ford said the military balance is turning against the Assad regime, which has lost some critical strategic locations such as the borders with Turkey and Iraq. The ambassador also said there has been heavy fighting in Damascus "right up close to where the president lives."

Ford said Iran is increasing its military assistance to Assad's regime and the outside help has persuaded him that he can prevail.

"I think today he still thinks he can win militarily with help from Russia, from Iran, from Lebanese Hezbollah," Ford said. "But I think he also must understand as his windows rattle, because the fighting is getting closer, he must be thinking about whether or not his calculations are correct."

Ford was pressed repeatedly about what military action the United States might take but declined to speculate at the public hearing. Lawmakers uneasy with military involvement – or even the prospect of arming the opposition – reflected a war-weariness after more than a decade of conflicts in Iraq and Afghanistan.

Republican Rep. Scott Perry of Pennsylvania, who noted the 10th anniversary of the invasion of Iraq this week, repeatedly tried to get Ford to elaborate for Congress and the American people about what could happen next in Syria if chemical weapons were used.

Ford declined. Perry, alluding to Iraq, said, "We don't want the current administration making the mistake of past administrations."

In fact, no consensus has emerged in Congress about what further steps should be taken to break the stalemate in Syria. Some, such as Republican Sens. John McCain and Lindsey Graham, favor strikes on Syrian air defenses, establishment of a no-fly zone and arming the opposition.

Others, like Rep. Ileana Ros-Lehtinen, R-Fla., said Wednesday that providing weapons to the Syrian opposition risks having the weapons fall into the wrong hands.

"The unknown can be dangerous and the vetting of the opposition is not enough when it comes to providing lethal aid that could be used against our allies, such as Israel, or the United States in a post-Assad era," she said.

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Associated Press writer Donna Cassata in Washington contributed to this report.

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  • March 2011: 120

    Thousands of Syrians rally to show their support for President Bashar al-Assad, who is facing unprecedented domestic pressure amid a wave of dissent, in Damascus on March 29 2011. (ANWAR AMRO/AFP/Getty Images)

  • April 2011: 820

    A protestor burns a portrait of Syrian President Bashar al-Assad during a demonstration after Friday prayers on April 29 2011 in Istanbul against the regime of al-Assad and the deadly crackdown on opposition protests. (BULENT KILIC/AFP/Getty Images)

  • May 2011: 850

    A veiled woman takes part in a protest calling on Syria's President Bashar Assad to step down, in front of the United Nations headquarters in Amman, on May 21 2011. (KHALIL MAZRAAWI/AFP/Getty Images)

  • June 2011: 1,000

    Syrian refugees arrive to a makeshift camp in the northern city of Idlib, in Syria, on June 13 2011. (MUSTAFA OZER/AFP/Getty Images) <em><strong>CORRECTION:</strong> An earlier version of this caption placed the city of Idlib in Turkey. Idlib is in Syria. </em>

  • July 2011: 1,600

    Thousands of pro-regime Syrians wave their national flag and portraits of President Bashar al-Assad during a rally in Damascus on July 17 2011. (LOUAI BESHARA/AFP/Getty Images)

  • August 2011: More than 2,000

    People hold pictures of Syrian president Bashar al-Assad and fallen Libyan strongman Muammar Gaddafi as they take part in a demonstration gathering activists opposed to Syria's regime of President Bashar al-Assad on August 28 2011 at Taksim Square in Istanbul. (BULENT KILIC/AFP/Getty Images)

  • September 2011: 2,700

    A national flag hangs on a statue of Syria's late president Hafez al-Assad at the entrance of the flashpoint city of Homs on August 30 2011, as rights activists reported widespread anti-regime protests across Syria on the first day of the feast marking the end of Ramadan. (JOSEPH EID/AFP/Getty Images)

  • October 2011: 3,000

    Supporters of Syrian President Bashar al-Assad wave Syrian flags during a pro-regime rally in Damascus on October 12, 2011. Assad's regime is facing international pressure amid a violent crackdown on anti-government protests that broke out in March across Syria. (LOUAI BESHARA/AFP/Getty Images)

  • November 2011: More than 4,000

    Syrian soldiers carry on November 26, 2011 the coffin of a comrade reportedly killed in an ambush by an armed group in the flashpoint Syrian city of Homs. (AFP/Getty Images)

  • December 2011: More than 5,000

    Free Syrian Army captain identified as Ahmed al-Arabi sits in a safe house near Wadi Khaled on the Lebanese-Syrian border on December 30 2011. (Si Mitchell/AFP/Getty Images)

  • January 2012: 7,100

    Free Syrian Army fighters take position in a house on the Lebanese-Syrian border prior to a nighttime operation on January 2 2012. (Si Mitchell/AFP/Getty Images)

  • February 2012: 7,500

    A Turkish journalist in Ankara, holds pictures of two journalists, French photojournalist Remi Ochlik and Sunday Times correspondent Marie Colvin, killed in an alleged rocket attack by Syrian regime forces against a makeshift opposition media center in the besieged city of Homs in Syria on February 22 2012. (ADEM ALTAN/AFP/Getty Images)

  • March 2012: More than 8,000

    A defected Syrian soldier, now a member of the Free Syrian Army, stands outside a mountain outpost near the village of Janudieh in the northern province of Idlib on March 20 2012. (STR/AFP/Getty Images)

  • April 2012: Close to 9,000

    Syrians pray over the bodies of Syrian violence victims at a funeral in the northwestern town of Kafr Zeta on April 10, 2012. (AFP/Getty Images)

  • May 2012: More than 9,000

    A general view shows the Syrian flag flying next to destruction in the Bab Amro neighbourhood of Homs on May 2 2012. (JOSEPH EID/AFP/GettyImages)

  • June 2012: 14,000

    A Syrian man carries a wounded girl next to Red Crescent ambulances following an explosion that targeted a military bus near Qudssaya, a neighbourhood of the Syrian capital, on June 8, 2012. (AFP/GettyImages)

  • July 2012: 19,000

    Members of Jihadist group Hamza Abdualmuttalib train near Aleppo on July 19, 2012. Rebels seized control of all of Syria's border crossings with Iraq on July 19. (BULENT KILIC/AFP/GettyImages)

  • August 2012: 23,000

    A Syrian boy whose family has been displaced due to fighting between rebel fighterws and Syrian government forces is seen near the Syrian border with Turkey on August 25, 2012. (ARIS MESSINIS/AFP/GettyImages)

  • September 2012: 30,000

    A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in the northern city of Aleppo on September 14, 2012. (MARCO LONGARI/AFP/Getty Images)

  • October 2012: 36,000

    A Syrian boy plays on a destroyed tank near the rubble of a mosque that was destroyed during fighting between Syrian rebels and regime forces in the northern city of Azaz on September 23, 2012. (MIGUEL MEDINA/AFP/GettyImages)

  • November 2012: 40,000

    A walnut tree stripped of its branches stands in the rubble of the Kalat al-Numan citadel, originally built during the Roman era some 2000-years-ago, after allegedly being bombed several times by the Syrian air force on November 18, 2012, in Maaret Al-Numan in southern Idlib province. (John Cantlie/AFP/Getty Images)

  • December 2012: 60,000

    Smoke rises in the Hanano and Bustan al-Basha districts in the northern city of Aleppo on December 1, 2012 as fighting continues through the night. (Javier Manzano/AFP/Getty Images)

  • January 2013: 65,000

    A boy plays with a balloon in a Syrian refugees camp in Azaz, near the Turkish border, on January 10, 2013 after snow falls. (EDOUARD ELIAS/AFP/Getty Images)

  • February 2013: More than 70,000

    Syrian Zakia Abdullah sits on the rubble of her house in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. (Pablo Tosco/AFP/Getty Images)