U.S. officials said the growing intelligence pointed strongly toward Bashar Assad's government as the culprit — a claim Assad called "preposterous."
The U.S., along with allies in Europe, appeared to be laying the groundwork for the most aggressive response since Syria's civil war began more than two years ago. Obama has not yet decided how to respond to the use of deadly gases, officials said. The president said last year that type of warfare would cross a "red line."
Two administration officials said the U.S. was expected to make public a more formal determination of chemical weapons use on Tuesday, with an announcement of Obama's response likely to follow quickly. The officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.
On Monday, as he sought support from allies, Secretary of State John Kerry called the evidence of a large-scale chemical weapons attack "undeniable." And he said that international standards against chemical weapons "cannot be violated without consequences."
The Obama administration's tougher language marked the clearest justification yet for any U.S. military action in Syria, which most likely would involve sea-launched cruise missile attacks on Syrian military targets.
Hagel told BBC television on Tuesday that the Defense Department has "moved assets in place to be able to fulfill and comply with whatever option the president wishes to take."
The Navy has four destroyers in the eastern Mediterranean Sea within range of targets inside Syria. The U.S. also has warplanes in the region.
"We are ready to go," Hagel said.
Hagel said "to me it's clearer and clearer" that the Syrian government was responsible, but that the Obama administration was waiting for intelligence agencies to make the determination.
Hagel was interviewed during a visit to the Southeast Asian nation of Brunei. While there, Hagel spoke by phone about Syria with his counterparts from Britain and France. Hagel's press secretary, George Little, said, said Hagel "conveyed that the United States is committed to working with the international community to respond to the outrageous chemical attacks that have claimed the lives of innocent civilians in Syria."
In London, Prime Minister David Cameron recalled Parliament for an urgent discussion on a possible military response. Cameron said the crisis session will be held Thursday for a clear government motion and vote on the British response to a chemical weapons attack in Syria.
The British government said its military is drawing up contingency plans for a possible attack. Italy, meanwhile, is insisting that any strike must be authorized by the U.N. Security Council.
Assad was defiant. In an interview published Tuesday on the website of the state-run Syrian Arab News Agency, Assad accused the U.S. and other countries of "disdain and blatant disrespect of their own public opinion; there isn't a body in the world, let alone a superpower, that makes an accusation and then goes about collecting evidence to prove its point."
Assad warned that if the U.S. attacks Syria, it will face "what it has been confronted with in every war since Vietnam: failure."
The international community appeared to be considering action that would punish Assad for deploying deadly gases, not sweeping measures aimed at ousting the Syrian leader or strengthening rebel forces. The focus of the internal debate underscores the scant international appetite for a large-scale deployment of forces in Syria and the limited number of other options.
"We continue to believe that there's no military solution here that's good for the Syrian people, and that the best path forward is a political solution," State Department spokeswoman Marie Harf said. "This is about the violation of an international norm against the use of chemical weapons and how we should respond to that."
The Obama administration was moving ahead even as a United Nations team already on the ground in Syria collected evidence from last week's attack. The U.S. said Syria's delay in giving the inspectors access rendered their investigation meaningless and that the Obama administration had its own intelligence confirming chemical weapons use.
The U.N. team came under sniper fire Monday as it traveled to the site of the Aug. 21 attack and on Tuesday delayed a second inspection. A U.S. official said the U.N. team's delay would not affect the Obama administration's timeline for releasing its own intelligence assessments.
It's unlikely that the U.S. would launch a strike against Syria while the United Nations team is still in the country. The administration may also try to time any strike around Obama's travel schedule — he's due to hold meetings in Sweden and Russia next week — in order to avoid having the commander in chief abroad when the U.S. launches military action.
The president has ruled out putting American troops on the ground in Syria and officials say they are not considering setting up a unilateral no-fly zone.
On Capitol Hill, bipartisan support for a military response appeared to be building, with some key lawmakers calling for targeted strikes. A spokesman for House Speaker John Boehner said the Ohio Republican had "preliminary communication" with White House officials about the situation in Syria and a potential American response.
Speaking to reporters at the State Department on Monday, Kerry was harshly critical of chemical warfare.
"By any standard, it is inexcusable and — despite the excuses and equivocations that some have manufactured — it is undeniable," Kerry said, confirming the attack in the Damascus suburbs that activists say killed hundreds of people.
The U.S. assessment is based in part on the number of reported victims, the symptoms of those injured or killed and witness accounts. Administration officials said the U.S. had additional intelligence confirming chemical weapons use and would make that information public.
Officials stopped short of unequivocally stating that Assad's government was behind the attack. But they said there was "very little doubt" that it originated with the regime, noting that Syria's rebel forces do not appear to have access to the country's chemical weapons stockpile.
It's unclear whether Obama would seek authority from the U.N. or Congress before using force. The president has spoken frequently about his preference for taking military action only with international backing, but it is likely Russia and China would block U.S. efforts to authorize action through the U.N. Security Council.
More than 100,000 people have died in clashes between forces loyal to Assad and rebels trying to oust him from power over the past two and a half years. While Obama has repeatedly called for Assad to leave power, he has resisted calls for a robust U.S. intervention, and has largely limited American assistance to humanitarian aid. The president said last year that chemical weapons use would cross a "red line" and would likely change his calculus in deciding on a U.S. response.
Obama took little action after Assad used chemical weapons on a small scale earlier this year and risks signaling to countries like Iran that his administration does not follow through on its warnings.
Officials said it was likely the targets of any cruise-missile attacks would be tied to the regime's ability to launch chemical weapons attacks. Possible targets would include weapons arsenals, command and control centers, radar and communications facilities, and other military headquarters. Less likely was a strike on a chemical weapons site because of the risk of releasing toxic gases.
Military experts and U.S. officials said Monday that the precision strikes would probably come during the night and target key military sites.
AP National Security Writer Robert Burns contributed to this report from Bander Seri Begawan, Brunei