Obama spoke with British Prime Minister David Cameron, a top US ally, and agreed that chemical weapon use by Syrian President Syrian President Bashar al-Assad's forces would merit a "serious response", a spokesperson for the prime minister said in a statement.
Syrian opposition accounts that between 500 and well over 1 000 civilians were killed this week by gas in munitions fired by pro-government forces, and video footage of victims' bodies, have stoked demands abroad for a robust, US-led response after two and a half years of international inaction on Syria's conflict.
Syria sought to avert blame by saying its soldiers had found chemical weapons in rebel tunnels. US Secretary of State John Kerry called his Syrian counterpart on Thursday to chide the government for not allowing UN inspectors access to the site.
Obama has been reluctant to intervene in Syria's civil war, but reports of the killings near Damascus have put pressure on the White House to make good on the president's comment a year ago that chemical weapons would be a "red line" for the United States.
The United States is repositioning naval forces in the Mediterranean to give Obama the option for an armed strike.
The White House declined to list what options were discussed on Saturday and said Washington was still gathering details about the attack.
"In coordination with international partners and mindful of the dozens of contemporaneous witness accounts and record of the symptoms of those killed, the US intelligence community continues to gather facts to ascertain what occurred," it said in a statement.
American and European security sources have said US and allied intelligence agencies made a preliminary assessment that chemical weapons were used by Syrian forces in the attack. The United Nations has requested access to the site.
Obama spoke to Cameron after the White House meeting. A spokesperson for the British prime minister said the two men noted increasing signs of Syrian government culpability.
"They are both gravely concerned by the attack that took place in Damascus on Wednesday and the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people," the spokesperson said.
Cameron also spoke to Canadian Prime Minister Stephen Harper. Harper spoke to French President François Hollande.
'Silence and inaction'
Obama said in a CNN interview broadcast on Friday that chemical weapon use on a large scale would start "getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region".
But Americans strongly oppose US intervention in Syria's civil war and believe Washington should stay out of the conflict even if reports that Syria's government used deadly chemicals to attack civilians are confirmed, according to a new Reuters/Ipsos poll.
Syrian state television said soldiers found chemical materials on Saturday in tunnels that had been used by rebels, rejecting the blame for carrying out a nerve gas attack.
The state news agency, Sana, said soldiers had "suffered from cases of suffocation" when rebels used poison gas "as a last resort" after government forces made "big gains" against them in the Damascus suburb of Jobar.
The leader of the opposition Syrian National Coalition, Ahmad al-Jarba, and the head of the rebel Free Syrian Army, General Salim Idriss, denied on Saturday that rebels had used chemical weapons.
Jabra said the "most important cause" of the attack was the silence and inaction of the international community, especially the West.
Kerry called Syrian Foreign Minister Walid al-Moualem on Thursday and told him the Damascus government should have let UN inspectors have access to the site of the alleged gas attack, a State Department official said.
"If, as they claimed, the Syrian regime has nothing to hide, it should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence," the official said, referring to Kerry's message in the call.
Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy, said there was growing consensus in the West that Assad's government was responsible.
"It's very clear, I think, that the US and the Western governments think that the regime did it," he said.
"Whether their response would immediately be military or not, I don't know. I suspect that first they're probably going to push for diplomacy, but probably with a pretty short fuse."
Kerry made a series of diplomatic calls to counterparts in the region, including Saudi Arabia, Jordan, and Turkey on Saturday.
In the most authoritative account so far, the medical charity Médecins Sans Frontières said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3 600 admissions with nerve gas-type symptoms.
A senior UN official arrived in Damascus to seek access for inspectors to the site of last Wednesday's attack.
Major world powers — including Russia, Assad's main ally which has long blocked UN-sponsored intervention against him — have urged the Syrian leader to cooperate with UN chemical weapons inspectors already in Damascus to pursue earlier allegations.
But Russia said the rebels were impeding an inquiry and that Assad would have no interest in using poison gas for fear of foreign intervention.
Alexei Pushkov, pro-Kremlin chairperson of the international affairs committee in Russia's lower house of Parliament, said: "In London, they are 'convinced' that Assad used chemical weapons, and earlier they were 'convinced' that there were weapons of mass destruction in Iraq. It's the same old story."
'Range of options'
The list of participants in the White House meeting underscored its importance. They included Vice President Joe Biden, national security adviser Susan Rice, CIA director John Brennan, chairman of the Joint Chiefs of Staff Martin Dempsey, and Samantha Power, the US representative to the United Nations.
Aides said Kerry, who is on vacation in Massachusetts, and Secretary of Defence Chuck Hagel, who is travelling in Asia, both participated remotely.
Administration officials were cautious in describing the content of the discussions and warned against expectations of a decision on 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," a White House official said before the meeting.
"Once we ascertain the facts, the president will make an informed decision about how to respond," a White House official said.
Hagel said on Friday the Pentagon was taking measures to prepare for all options. "And that requires positioning our forces, positioning our assets, to be able to carry out different options — whatever options the president might choose," he said without elaborating.
A defence official, speaking on condition of anonymity, said the US Navy would expand its presence in the Mediterranean to four destroyers from three.
Iranian President Hassan Rouhani weighed in on the issue for the first time, saying chemical weapons had killed people in Syria, its ally. Although Rouhani stopped short of saying who he thought had used the weapons, Iran's Foreign Ministry said evidence pointed to rebels fighting Syrian President Bashar al-Assad.
The US Central Command and the Jordanian armed forces were planning to host a meeting of regional defence chiefs from Sunday to Tuesday in Jordan. The group will discuss "the region's dynamic security environment". The meeting was scheduled in June and not called in response to the recent attacks in Syria, a Pentagon spokesperson said. – Reuters