The United States is repositioning naval forces in the Mediterranean and is ready to act if Barack Obama calls for a strike against Syria, Defence Secretary Chuck Hagel indicated.
Obama has said Washington must be wary of costly and difficult foreign interventions as calls for action against the regime of Syrian President Bashar al-Assad mount over his alleged use of chemical weapons.
But US commanders have prepared a range of options for Obama, Hagel said. "The Defence Department has a responsibility to provide the president with options for all contingencies," he explained.
Yesterday Obama met with security aides to discuss a response.
"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.
A defence official said the USS Mahan, a destroyer armed with cruise missiles, had finished its deployment and was due to head back to its home base in Norfolk, Virginia. But the commander of the US Sixth Fleet had decided to keep the ship in the region.
Obama has been hesitant to intervene in Syria's civil war.
But American and European security sources said that US and allied intelligence agencies had made a preliminary assessment that chemical weapons were used by Syrian forces in the attack near Damascus last week.
The Paris-based humanitarian aid group, Doctors Without Borders, said that three hospitals it supports in the Damascus region reported receiving roughly 3,600 patients who showed "neurotoxic symptoms" on Wednesday morning. It said 355 of them died.
The Syrian government has denied using chemical weapons, and state television said soldiers entering a rebel-held area have found "barrels filled with highly dangerous toxic and chemical agents" as well as gas masks.
International powers - including Russia, which has long shielded Assad from United Nations action - have urged Assad to co-operate with a UN inspection team that arrived on Sunday to pursue earlier allegations of chemical warfare.
But US and foreign diplomats said Russia's move did not reflect any shift in its backing of Assad or its resistance to punitive measures in the Security Council.
German Chancellor Angela Merkel criticised China and Russia for blocking a Western-backed UN demand to give inspectors unfettered access.
"Unfortunately, the opposition of Russia and China thwarted a formal statement by the UN Security Council calling for them to be granted full access," Merkel told Focus magazine.
UN Under Secretary General Angela Kane arrived in Damascus, tasked by Secretary General Ban Ki-moon for talks about the terms of an inquiry.
US security aides are also studying the Nato air war in Kosovo as a possible blueprint for acting without a UN mandate.
And with Russia still likely to veto any military action in the Security Council, Obama appears to be wrestling with whether to bypass the UN.
He warned doing so would require a robust international coalition and legal justification.
But other Western officials have been less cautious. British Foreign Secretary William Hague said: "I know some people would like to say that this is some kind of conspiracy brought about by the opposition in Syria.
"I think the chances of that are vanishingly small, and so we do believe that this is a chemical attack by the Assad regime."
Hague did not speak of using force, as France has, if the government was found to have been behind the attack. But he said it was "not something a humane or civilized world can ignore".
Such statements carry echoes of Kosovo, where the Yugoslav government of Slobodan Milosevic brutally cracked down on ethnic Albanians in 1998 and 1999, leading the Clinton administration to decide to act militarily in concert with Nato allies.
The New York Times, Associated Press, Agence France-Presse, Bloomberg