"Once we establish the facts I have made clear that the use of chemical weapons is a game changer," Obama said in a joint news conference with Israeli Prime Minister Binyamin Netanyahu in Jerusalem.
Obama said he would not discuss any "next steps" until he knew all the facts but indicated any response would likely be coordinated among the international community.
"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," he said. "And the international community has to act on that additional information."
The Obama administration, which has been reluctant to involve U.S. forces in support of the Syrian opposition, has said it would intervene militarily in the 2-year-old civil war to stop the Assad regime from using chemical or biological weapons, or to stop it from transferring them to extremist groups.
"We know the Syrian government has the capacity to carry out chemical attacks" and some Assad-regime officials "have expressed a willingness to use chemical weapons," Obama said.
He added he was "deeply skeptical of any claim that, in fact, it was the opposition that used chemical weapons."
Shortly before Obama spoke, U.S. Ambassador to Syria Robert Ford told the House Foreign Affairs Committee in Washington the administration had no independent evidence chemical weapons had been used.
Israeli Justice Minister Tzipi Livni, sworn in Monday and among the Israeli officials who met with Obama in Jerusalem Wednesday, told CNN, "It's clear for us here in Israel that it's being used."
The allegations followed reports Tuesday of a chemical-weapons attack in the Syrian town of Khan al-Assal, southwest of Aleppo, that killed at least 19 civilians and left 69 others with breathing problems. Some were in critical condition, activists and rebels said.
Witnesses to the attack described a chlorine-like smell in the area.
The Syrian opposition and the regime of President Bashar Assad laid blame and accusations of chemical weapons use on the other, and both sides demanded an international investigation.
The umbrella Syrian National Coalition said it discovered there were actually two chemical-warfare strikes Tuesday, the second in suburban Damascus.
It condemned the attacks and said it held "the Assad regime fully responsible for shedding Syrian blood."
At the United Nations, Syrian Ambassador Bashar Jaafari told reporters Damascus asked Secretary-General Ban Ki-moon to investigate the attacks to corroborate Syrian regime claims insurgents -- not regime forces -- used the weapons.
He said his letter to Ban from the Syrian Foreign Ministry Wednesday sought a "specialized, independent and neutral technical mission to investigate the use by the terrorist groups operating in Syria of chemical weapons yesterday against civilians."
"Terrorist groups" is the regime's term for rebel fighters.
Ban's office said it was studying the request.
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