Washington has noted that Syria offered to let UN inspectors view the site of the alleged attack, but suggested it was too little, too late, said one official, speaking on condition of anonymity.
“At this juncture, the belated decision by the regime to grant access to the UN team is too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime’s persistent shelling and other intentional actions over the last five days,” the official said.
If confirmed, the attack would mark the deadliest use of chemical agents since Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
US Defence Secretary Chuck Hagel said earlier Sunday the US military was “prepared to exercise whatever option” against Syria but intelligence was still being evaluated.
On a visit to Malaysia, Hagel said the US defence department had prepared “options for all contingencies” at Obama’s request.
Obama had said a year ago that the use of chemical weapons by Assad’s forces was a “red line” that could trigger Western intervention.
On Sunday, a strident warning came from Washington’s arch foe Iran.
“If the United States crosses this red line, there will be harsh consequences for the White House,” armed forces deputy chief of staff Massoud Jazayeri said, without elaborating.
The Arab League is to meet on Tuesday to discuss the alleged use of chemicals, the bloc’s deputy chief Ahmed Ben Helli said.
In Israel, President Shimon Peres called for international efforts to “take out” chemical weapons in Syria as Prime Minister Benjamin Netanyahu said Israel would pull the “trigger” if needed to protect its people.
More than 100,000 people have been killed in Syria since an uprising against Assad’s rule flared in March 2011, the UN says.
In the latest eruption of violence, the governor of Hama province in central Syria was killed in a car bombing on Sunday, state television reported, in an attack it blamed on rebels.