The U.S. millitary is "ready to go" if President Barack Obama orders action over a chemical weapons attack in Syria, Defense Secretary Chuck Hagel told the BBC on Tuesday.
Oil prices jumped, and U.S. stock futures fell.
(Read more: US stocks seen down on Syria fears)
Britain's Prime Minister David Cameron has recalled parliament to vote on Syria on Thursday, and MPs are saying this could mean a strike will come as early as Friday or the weekend.
"We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take," Hagel said during a trip to Brunei, according to a partial transcript provided by the BBC. Asked if the U.S. military was ready to respond just "like that," Hagel said: "We are ready to go, like that."
Western powers, meanwhile, told the Syrian opposition to expect a strike against President Bashar al-Assad's forces within days, according to sources who attended a meeting between envoys and the Syrian National Coalition in Istanbul.
"The opposition was told in clear terms that action to deter further use of chemical weapons by the Assad regime could come as early as in the next few days, and that they should still prepare for peace talks at Geneva," one of the sources who was at the meeting on Monday told Reuters.