WASHINGTONThe 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.
U.S. intelligence agencies believe the main poison used was likely sarin, and estimate the death toll at 500 to 1,000 people, according to a senior U.S. official.
Syrian government officials have denied the Assad regime used chemical weapons, and accused their opponents of staging the attacks to provoke international action. Mr. Assad on Thursday said that "Syria will defend itself against any aggression," Syrian state media reported.
As support sagged abroad, the White House scrambled Thursday to shore up support at home by meeting with lawmakers and by providing them with some of the findings of the intelligence agencies.
The administration presented its case in a 90-minute conference call to congressional leaders Thursday evening.
Several lawmakers said they came away from the briefing with a conviction that the Assad regime was behind the attack, but that President Obama must do more to persuade the country military action is needed and to rally international support.
Rep. Eliot Engel (D., N.Y.) said officials told the lawmakers in the call that they had intercepted a phone call from a high level Syrian official discussing the attacks, which they said was proof the Assad regime was responsible for the chemical weapons attack.
A member of the House Intelligence Committee who had been briefed earlier on the intelligence said that the administration should wait until the U.N. inspectors have reported their findings.
"The evidence is substantial but I'm still very interested in the clinical work that is being done by the United Nations," said Rep. Adam Schiff (D., Calif.). "I would want to know what the answers are to the clinical piece before I would be ready to say that it's confirmed."
As of Thursday evening, neither Mr. Obama nor his aides had made a public case to support their claims about Mr. Assad's role in the attack. The White House has said it would make some findings public by the end of this week.
Friday morning in the Philippines, Defense Secretary Chuck Hagel said despite Britain's rejection of support, the U.S. would continue to work to build an international coalition. "Our approach is to continue to find an international coalition that will act together," Mr. Hagel said.
The U.S. military could still work with France, whose president, François Hollande, doesn't need parliamentary approval to join in a military mission, and who reiterated his support for action on Thursday. Most opposition leaders in France have said they would support strikes.
German Chancellor Angela Merkel, however, told Mr. Obama in a phone call that any action should be addressed by the Security Council, Ms. Merkel's spokesman said. The two leaders agreed, however, that the Aug. 21 attack represented a "grave violation of international law," the spokesman said.
If Mr. Obama acts alone, it would mark an about-face for a president who as recently as a week ago stressed the dangers of military action without U.N. backing.
White House officials on Thursday signaled a desire to act quickly in Syria, on the U.S.'s own timetable and unilaterally, if necessary.
They cited a concern that waiting longer would inflame debates in the U.S. and Europe, while providing Syria more of an opportunity to cover its tracks and giving Syria's allies time to whip up international opposition to U.S. strikes.
On Thursday, U.N. inspectors continued work on sites of suspected chemical attacks in Syria, but they have yet to make any findings public. The U.N. plans to pull the inspectors out on Saturday, a move that could make it easier for the U.S. to make a final decision to act.
A meeting of the permanent members of the U.N. Security Council in New York on Thursday ended with no sign of progress on an agreement.
U.S. officials have said the intelligence is clear-cut and that they don't need to await the U.N. investigators' findings before deciding how to proceed.
In recent days, U.S. intelligence agencies and the Pentagon have watched with alarm as Mr. Assad has taken advantage of the Western deliberations to spread out his forces, complicating U.S. planning for strikes.
"We know [Assad] has been dispersing assets," said a senior U.S. official briefed on the intelligence.
U.S. officials said Mr. Assad has moved assets such as military helicopters and artillery pieces around the country, forcing a U.S. recalibration of the possible military response.
If Mr. Obama sticks with what originally was a finite set of prospective military and intelligence targets, officials said, then cruise-missile strikes would cause less damage than originally intended because at least some of the targets have been taken out of the line of fire.
Officials said Mr. Obama could adjust to Mr. Assad's tactics by expanding the number of strikes to hit more targets, but doing so could increase the risk that U.S. cruise missiles will cause unintended damage, including civilian casualties, officials said.
Another senior official said the dispersal of Mr. Assad's military assets was "certainly detrimental" to target planning.
U.S. officials on Thursday defended the strength of the intelligence linking the Assad regime to the alleged Aug. 21 chemical attacks, but the Obama administration held off on releasing the unclassified intelligence.
One senior U.S. official cited "multiple pieces of evidence of regime involvement," adding: "Nobody's thinking that this is a rogue operation" by the Syrian unit that controls the regime's arsenal of poison gas.
But administration officials faced continued questioning Thursday about the strength of the intelligence that hasn't yet been made public, particularly whether the administration could avoid the pitfalls that led to the errant CIA assessment in 2003 that Iraq had weapons of mass destruction.
The Syria intelligence case is "in no way analogous" to 2003, said State Department spokeswoman Marie Harf. The difference, she said, is that there is no question that the Assad regime has chemical weapons and has used them.
"This is a response to something that has already happened," she said. "We are not going to repeat the same mistakes of the Iraq war."
The U.S. response to the Syrian civil war has fueled criticism by some politicians of the White House, going back to Mr. Obama's 2012 declaration that Syria's use of chemical weapons would cross a U.S. red line.
"It is troubling that the president apparently did not have a thoroughly vetted plan for responding," when he issued that warning, said Sen. Susan Collins (R., Maine), a member of the Senate Intelligence Committee.
Sen. Ron Johnson (R., Wis.) said he believed that the evidence was strong, but that the president needed to make the case to the American public and get congressional approval before taking action.
"I think it is a good thing that we are not going to rush into it," said Mr. Johnson.
In Damascus, as the prospect of a U.S. strike loomed on Thursday, there was an increased presence by soldiers, security force members and members of pro-regime militias on the streets of the Syrian capital.
Many amassed at government offices, ministries, schools and other public installations. Some streets were blocked off with buses and cars.
Security force members checked vehicles parked in the area around the Damascus provincial government headquarters in the center of the city.
Tensions were high in some parts of the city that are home to several military and security facilities. At one busy intersection, a soldier was seen shouting and firing his weapon in the air to disperse a traffic jam.
Later on Thursday, regime supporters drove around the city honking horns and waving large Syrian flags from their vehicles as show of support for Mr. Assad against what they see as a foreign invasion.
Some flags were emblazoned with a portrait of the president. A popular patriotic song, "Assad You Are Loved by Millions" blared from some car stereos.
Sam Dagher in Damascus and Cassell Bryan-Low
in London contributed to this article.