US President Barack Obama has ordered US intelligence to urgently probe claims that Syrian forces launched a chemical attack on civilians, including children, aides said.
The Obama administration is facing rising political pressure for a tough response to the attack, which could have again infringed US "red lines" against chemical weapons use by President Bashar al-Assad's regime.
"Right now, we are unable to conclusively determine CW [chemical weapons] use," State Department spokesman Jen Psaki said on Thursday.
But she said the administration had been focused continually since reports of the attack broke on Wednesday on efforts to "nail down the facts".
"If these reports are true, it would be an outrageous and flagrant escalation of [the] use of chemical weapons by the regime," Psaki said.
Officials said they could not yet be sure the deadly arms had killed as many as 1,300 people outside Damascus, even though US ally France said it was likely they were used.
The United States held a series of diplomatic talks on Thursday to discuss possible new action against the Syrian government amid mounting international concern over alleged chemical weapons attacks.
John Kerry, the US secretary of state, discussed the episode with key counterparts, including Syria's main opposition leader Ahmad Assi Jarba and French Foreign Minister Laurent Fabius.
Kerry also called top officials at the EU, the UN and in Jordan, Qatar and Turkey.
|France warns Syria of forceful response
Obama has a range of options at his disposal and would discuss a response with his top national security aides, Psaki said.
The president has been loath to order US military action to protect civilians in Syria, fearing being drawn into a vicious civil war, just soon after he extracted US troops from Iraq.
But revulsion over video footage and photos of dead children blanketing the US media has reopened the debate about a policy Obama allies see as prudent but critics brand as weak.
The White House concluded earlier this year that Syria had crossed Obama's red line against the use of chemical weapons, but opted not to take military action.
Instead, it decided to directly arm selected Syrian rebel groups battling Assad - but has declined to publicly specify the extent of the support.