US President Barack Obama has warned Syria that using chemical weapons would be a "game changer," as he faces rising pressure at home and abroad to intervene in the country's bloody civil war.
But speaking Friday, a day after US officials said they suspected the use of the deadly agent sarin in small-scale attacks, Obama warned that Washington must act prudently, and establish exactly if, how and when such arms were used.
Obama, who had previously told Syrian President Bashar al-Assad that the use of chemical weapons would cross a "red line," promised a "vigorous" US and international probe into the latest reports.
But he appeared wary of launching military action based on initial intelligence reports of chemical weapons use.
Obama did reiterate that the use of chemical weapons would be "a game changer," as he met Jordan's King Abdullah II in the Oval Office.
"I think all of us, not just in the United States but around the world, recognise how we cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations," he said.
"We have to act prudently. We have to make assessments deliberately," he cautioned.
The spectre of the invasion of Iraq, which led to a eight-year war that killed tens of thousands of Iraqis and nearly 4,500 US soldiers, looms large.
The 2003 invasion was justified on the basis of claims that Saddam Hussein's regime had weapons of mass destruction, which turned out to be false.
"We have seen very bad movies before when intelligence is perceived to have driven policy decisions that in the full light of day have proven wrong," a US defence official said on condition of anonymity.
Adding to the political heat on Obama, the Syrian opposition urged the UN Security Council to take immediate steps, possibly even by imposing a no-fly zone.
And British Prime Minister David Cameron said the growing evidence that Assad had turned chemical agents on his own people was "extremely serious".
The fighting in Syria, which the UN says has killed more than 70,000 people since March 2011, showed no signs of abating on Saturday, with the Syrian Observatory for Human Rights reporting fresh fighting in central Hama.
The watchdog said insurgents and Assad's troops were clashing near the central prison, and also reported regime air raids on Douma, a rebel-held town just outside the capital Damascus.
In the southern province of Daraa, the Britain-based group said helicopter gunships had opened fire on Naeema, and shelled the towns of Saida and Tafas.
US claims that Damascus has likely used chemical weapons against rebel forces opens a potentially serious new dimension to the Syria conflict.
White House spokesman Jay Carney said Friday the White House wanted to establish "credible and corroborated" facts to determine whether the red line has been crossed, stressing "all options remain on the table".
Both regime and rebels have accused each other of chemical weapon use, and the UN has set up a team to investigate the claims, but Damascus has refused to allow access to all the sites in question.
A UN spokesman said investigators from the world body had started collecting evidence outside Syria on the suspected use of chemical weapons, and Syria's opposition has urged UN Security Council action.
"Should it find the regime used such weapons, it must act immediately, at least by imposing a no-fly zone," an official from the main opposition National Coalition told AFP on condition of anonymity.
The Security Council agreed a no-fly zone over Libya during the 2011 uprising that ousted long-time dictator Moamer Kadhafi, but it has been deadlocked on Syria for more than two years, with permanent members Russia and China vetoing several draft resolutions to impose sanctions on the Assad regime.
The opposition National Coalition has accused the regime of using chemical weapons in the northern province of Aleppo, in Homs in central Syria, and in rebel-held areas near Damascus.