Syria yesterday agreed to let chemical weapons experts visit the site and the UN said it would start its investigation today - a move the US said had come too late.
The office of UN Secretary-General Ban Ki-moon said in a statement that Syria had promised to observe a ceasefire at the site in the suburbs of Damascus while a UN team begins "on-site fact-finding activities".
The experts arrived in Damascus three days before a mass poisoning killed many hundreds of people on Wednesday in what appears to have been the world's worst chemical weapons attack in 25 years.
"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts and other facts gathered by open sources, the US intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident,"said a senior official in the US administration.
"We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons."
The official made clear the Syrian government's agreement to let UN inspectors visit the site was inadequate.
"At this juncture, any belated decision by the regime to grant access to the UN team would be considered 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.
The US official said the administration had seen reports that Syria would provide access, but said that if the government had nothing to hide, it would have allowed investigators to visit the site five days ago.
Meanwhile, Russia welcomed the move by the Syrian Foreign Ministry and warned against the "tragic mistake" of jumping to conclusions before the investigation is completed.
Senior US lawmakers have called for limited US military action in response to the alleged chemical weapons attack.
"I certainly would do cruise missile strikes," said Eliot Engel, the ranking Democrat on the House Foreign Affairs Committee.
Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee, told Fox News he thought Obama would "respond in a surgical way".
"I hope the president as soon as we get back to Washington will ask for authorisation from Congress to do something in a very surgical and proportional way," he said.
Two other Republican senators, John McCain and Lindsey Graham, issued a statement calling for "stand-off" strikes, such as by cruise missiles, to degrade the government's air power and help establish "safe areas" on the ground.
David Cameron and Mr Obama had earlier pledged a "serious response The Prime Minister and the US President discussed the growing crisis by telephone on Saturday night and have ordered officials to examine all options.
In response, Syria had warned the United States against any military action. "US military intervention will create a very serious fallout and a ball of fire that will inflame the Middle East," Syrian Information Minister Omran Zoabi was quoted as saying.
Mr Obama authorised sending US weapons to Syrian rebels in June, but shipments were delayed due to fears that radical Islamist groups in the opposition could gain further ground in Syria and become a threat to the West.