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.
The meeting at a hotel in downtown Istanbul was between senior figures of the Syrian National Coalition, including its president Ahmad Jarba, and envoys from 11 core "Friends of Syria" alliance members, that included U.S. envoy Robert Ford, the top U.S. official handling the Syria file, the sources said.
Facing Russian and Chinese disapproval that could dampen prospects for proposed peace talks in Geneva, Assad's foes have vowed to punish a poison gas attack in some rebel-held districts of Damascus on Aug. 21 that killed hundreds.
U.N. experts trying to establish what exactly happened in the attack were finally able to cross the front line on Monday to see survivors—despite being shot at in government-held territory. But they put off a second visit until Wednesday.
Hagel's comments came a day after Secretary of State John Kerry laid the groundwork for possible military action against the Syrian government by saying Obama believed there needed to be "accountability" for the use of chemical weapons.
Hagel said the United States would have intelligence to present "very shortly" about the attack. But he noted after calls with his British and French counterparts that there was little doubt among most U.S. allies that "the most base ... international humanitarian standard was violated."
—By Reuters with CNBC.com.