WASHINGTON, District of Columbia - US President Barack Obama and British Prime Minister David Cameron inched closer Saturday to attributing blame for a massive chemical weapons attack near Damascus to Bashar al-Assad's armed forces.
A Downing Street statement said the US and British leaders "are both gravely concerned by... increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people."
"The fact that President Assad has failed to cooperate with the UN suggests that the regime has something to hide," it added, noting that "significant use of chemical weapons would merit a serious response from the international community."
Obama and Cameron spoke by phone and will "continue to consult closely" regarding the reported massacre, a more circumspect White House statement said, after the US president met with his top security aides.
The Syrian government and its foes, meanwhile, accused each other of using chemical weapons, as Doctors Without Borders said 355 people had died of "neurotoxic" symptoms stemming from Wednesday's incident, with thousands more being treated in hospital.
If confirmed, the attack would be the deadliest use of chemical agents since Saddam Hussein gassed Iranian troops and Kurdish rebel areas in northern Iraq in the 1980s.
Obama's talks with security advisers came a day after Defence Secretary Chuck Hagel said the Pentagon had presented options to the president and strongly suggested US forces were being moved ahead of any possible decision on taking action in Syria.
So far, despite the reports of Wednesday's chemical attack on rebel-held areas near Damascus, Obama has continued to voice caution, warning that a hasty military response could have unforeseen consequences, including embroiling the United States in another prolonged Middle East conflict.
"The president has directed the intelligence community to gather facts and evidence so that we can determine what occurred in Syria. Once we ascertain the facts, the president will make an informed decision about how to respond," a White House official said earlier Saturday.
"We have a range of options available, and we are going to act very deliberately so that we're making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria."