Defence Secretary Chuck Hagel says the US military is ready to take action against Syrian forces if ordered, but stresses that the administration is still evaluating claims of a chemical weapons attack.
US President Barack Obama "has asked the Defense Department to prepare options for all contingencies. We have done that," Hagel told reporters in the Malaysian capital Kuala Lumpur.
"Again, we are prepared to exercise whatever option, if he decides to employ one of those options."
He said the US and its allies were assessing intelligence on allegations that President Bashar al-Assad's forces unleashed chemical weapons in an attack near Damascus last week.
"I wouldn't go further than that, until we have more intelligence based on facts," Hagel said.
Hagel spoke after a meeting with his Malaysian counterpart, Hishammuddin Hussein, as he started a week-long Southeast Asia tour to stress the country's much-touted renewed focus on the Asia-Pacific region.
But his attention has been diverted by crises in Syria and Egypt, where security forces have cracked down on supporters of ousted Islamist president Mohamed Morsi.
Hagel phoned Egypt's army chief from Kuala Lumpur to appeal for a peaceful resolution of political conflicts there, and has been in touch with top US security advisers on Syria.
Hagel reiterated the US military had presented a range of military options to President Barack Obama over the apparent chemical attack that has sparked international revulsion.
On his flight to Malaysia from Hawaii, he told reporters the US military was moving forces into place as needed, amid speculation Washington DC might opt for cruise missile strikes against Assad's regime.
Obama and British Prime Minister David Cameron appeared to move closer to pinning blame on Assad's 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 Syrian government and rebel forces have accused each other of using chemical weapons.
Doctors Without Borders said 355 people had died of "neurotoxic" symptoms stemming from Wednesday's incident.
The United Nations says more than 100,000 people have been killed in the Syrian conflict.
Hagel later gave a speech in which he underlined his country's bid to rebalance its strategic focus towards the Asia-Pacific.
The tilt to Asia is seen as based in part on the region's growing economic importance and to counterbalance the expanding military might of China.
The United States was increasing the number of naval ships in the Asia-Pacific and pursuing agreements to allow vessels, aircraft and troops to rotate through key ports and airfields while avoiding permanent US bases, an official said.
After his two-day stop in Malaysia, Hagel heads to Indonesia and then Brunei for a regional defence gathering that will include China.
