US President Barack Obama met with security aides Saturday to discuss a response to Syria's alleged use of chemical weapons after the Pentagon said it was preparing for possible military action.
Obama's National Security Council meeting came as Doctors Without Borders (MSF) reported that more than 350 people suffering "neurotoxic" symptoms had died following the reported attack.
The White House said the US intelligence community was still working to determine what had occurred but was "mindful of the dozens of contemporaneous witness accounts and record of the symptoms of those killed.
"The President also received a detailed review of a range of potential options he had requested be prepared for the United States and the international community to respond to the use of chemical weapons," a statement said.
Separately Obama also talked with British Prime Minister David Cameron to discuss possible responses by the international community, the White House said.
"The United States and UK stand united in our opposition to the use of chemical weapons," a White House press statement said.
US Defense Secretary Chuck Hagel on Friday said the military had presented options to Obama and was moving forces into place ahead of any possible decision.
Despite the reports of a massive chemical attack on rebel-held areas near Damascus, Obama has continued to voice caution, warning that a hasty military response could have unforseen consequences, including embroiling the United States in another prolonged Middle East conflict.
"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," a White House official said earlier.
Obama is under mounting pressure to act following reports of the alleged chemical weapons attack near Damascus on Wednesday, which Syrian opposition groups say killed as many as 1,300 people.
Doctors without Borders (MSF) said Saturday that three Syrian hospitals it works with received 3,600 patients displaying "neurotoxic symptoms," 355 of whom had died, following the alleged attack.
If confirmed, this would be the deadliest use of chemical agents since Saddam Hussein gassed Iranian troops and Kurdish rebel areas in northern Iraq in the 1980s.
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The Syrian government has denied using chemical weapons, and on Saturday state television said soldiers entering a rebel-held area had "suffocated" on poison gases deployed by "terrorists."
Obama warned a year ago that the use of chemical weapons by Syrian President Bashar al-Assad's forces was a "red line" that could bring about a more strident Western intervention in the more than two-year civil war.
US commanders have prepared a range of options for Obama if he chooses to proceed with military strikes against Damascus, Hagel told reporters during a visit to Southeast Asia.
"The Defense Department has a responsibility to provide the president with options for all contingencies," Hagel said.
"And that requires positioning our forces, positioning our assets to be able to carry out different options -- whatever the president might choose."
The New York Times cited a senior US administration official as saying Washington was looking at NATO's air war over Kosovo in 1999 as a blueprint for strikes on Syria without a UN mandate.
Russia, which has had a close military alliance with Damascus going back decades, has blocked UN action on Syria since the uprising against Assad erupted in March 2011.
Iran, another close ally of the Syrian government, has meanwhile blamed rebels for the chemical weapons attack and warned the West against any kind of intervention.
Previous reports of the use of chemical weapons on a small scale led Washington to announce in June that it would provide military aid to the rebels, but it is still unclear what that entails.
In an interview with CNN broadcast Friday, Obama said the alleged chemical attack appeared to have been a "big event, of grave concern," but remained cautious about any US military response.
"Sometimes what we've seen is folks will call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations," said Obama, who has spent much of his presidency winding down unpopular wars in Iraq and Afghanistan.
He warned that America could get "drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region."