US President Barack Obama warned Wednesday that if Syria let the "genie out of the bottle" by using chemical weapons in its civil war, it would unleash a serious international response.
Obama said in Israel that he was investigating reports that chemical weapons had been used in the vicious conflict in recent days -- following claims by each side that the other had fired off the deadly munitions.
But he said in his most expansive comments on the issue yet, that he was "deeply sceptical" that opposition forces had used such arms, saying it was well known that the government had control over chemical weapons stocks.
The president said at a news conference with Israeli Prime Minister Benjamin Netanyahu that he did not yet have the facts on the alleged use of the weapons.
"Once, we establish the facts, I have made clear that the use of chemical weapons is a game-changer," Obama said, warning that the use of such arms by Bashar al-Assad's forces would be a "grave and tragic" mistake.
"I won't make an announcement today about next steps because I think we have to gather the facts.
"But I do think that 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 has been reluctant to get the United States militarily involved in the Syrian conflict, and has blocked the dispatch of US lethal weapons and ammunition to rebels, despite fierce political pressure at home.
But Obama Wednesday denied Washington had done nothing, saying the United States had sent hundreds of millions of dollars in aid, and had worked with international partners to try and foster a political transition.
"This is not easy. When you start seeing a civil war that has sectarian elements to it, and you've got a repressive government that is intent on maintaining power... you end up seeing some of the devastation that you've been seeing."
Earlier, as Obama arrived in Israel on his first visit since moving into the White House in 2009, Israeli President Shimon Peres also warned of the danger posed by Syria's chemical weapons.
"Fortunately the Syrian nuclear capacity was destroyed but unfortunately the arsenal of chemical weapons remain. We cannot allow those weapons to fall into terrorists' hands -- it could lead to an epic tragedy," Peres said.
Syrian forces said that chemical weapons were used Tuesday in Khan al-Assal near Aleppo and the opposition has accused government forces of using chemical weapons.
Washington has said there is so far no evidence that rebels had fired chemical weapons, but said it would consult its allies on claims that the regime had used them.
Israel has consistently raised the alarm over Damascus's stockpiles of chemical weapons, raising fears they could fall into the hands of Lebanon's Hezbollah militia or other radical militant groups operating in Syria.