WASHINGTON—The Obama administration laid the groundwork for unilateral military action in Syria, a shift officials said reflected the U.K.'s abrupt decision not to participate and concerns that President Bashar al-Assad was using the delayed Western response to disperse his military assets.

The push for a quick international strike to punish Syria for what the U.S. said was a chemical-weapons attack appeared in disarray on Thursday, after British lawmakers defeated a government motion in support of military action.

But President Barack Obama is prepared to act without Britain, officials said, noting that unlike U.S. involvement in the 2011 military operation in Libya, the options under consideration in Syria are smaller-scale and wouldn't require a coalition to be effective.

"Here, what's being contemplated is of such a limited and narrow nature that it's not as if there's a similar imperative for bringing in different capabilities from different countries," a senior administration official said. "We believe it's important that there be diplomatic support from key allies, and we think we're getting that."

After a week of U.S. saber rattling that raised expectations about an imminent attack on Syria by a U.S.-led coalition, the White House had yet to release details about the intelligence, but said its findings conclusively showed the Assad regime used chemical weapons on a large scale against civilians last week.