The United States says there is "very little doubt" Syrian forces used chemical weapons on civilians and has dismissed an offer by Damascus for a UN team to view the attack site.
The comments, announced on Sunday, mark a significant escalation of a showdown over the horrific attack which killed 1300 people last week, at a time when Washington appeared to be positioning itself for possible military action.
Officials said President Barack Obama, who held crisis talks on Saturday with top aides, would make an "informed decision" about how to respond to an "indiscriminate" chemical weapons attack.
One has told Agence France-Presse that based on the reported number of victims and their symptoms, and US and foreign intelligence, "there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident".
The official, speaking on condition of anonymity, said Washington had noted that Syria had offered to let UN inspectors view the site of the alleged attack on Monday but suggested it was too little, too late.
"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the UN five days ago," the official said.
"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."
