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 U.S. intelligence community continues to gather facts to ascertain what occurred," it said in a statement.
American and European security sources have said U.S. 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.
"The fact that President Assad has failed to cooperate with the U.N. suggests that the regime has something to hide."
Cameron also spoke to Canadian Prime Minister Stephen Harper. Harper spoke to French President François Hollande.
Syria rejects blame
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 U.S. 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.
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 U.S. 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."
In the most authoritative account so far, the medical charity Medecins Sans Frontieres 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 U.N. 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 U.N.-sponsored intervention against him - have urged the Syrian leader to cooperate with U.N. 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 chairman 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."