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. As of Tuesday morning, officials said President Barack Obama had not yet decided how to respond to the use of deadly gases, a move he said last year would cross a red line.
The Obama administration has already said there is "undeniable" evidence of a large-scale chemical weapons attack in Syria last week. Officials had planned to make a more formal declaration linking Assad to the attack on Tuesday, but the release of the intelligence report appeared to have been pushed back until later in the week. It was unclear why the release was delayed.
A U.S. official said some of the evidence includes signals intelligence - information gathered from intercepted communications. The U.S. assessment is also based on the number of reported victims, the symptoms of those injured or killed and witness accounts.
The officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.
It's unlikely international military action would begin before Thursday. That's when British Prime Minister David Cameron will convene an emergency meeting of Parliament where lawmakers are expected to vote on a motion clearing the way for a British response to the chemical weapons attack.
International support was growing. In Paris, President Francois Hollande said Tuesday that France is "ready to punish those who took the heinous decision to gas innocents." And the Arab League, a 22-member body dominated by Saudi Arabia and Qatar, also called for justice, laying blame for the attack on the Syrian government.
Obama discussed the situation in Syria on Tuesday with Prime Minister Stephen Harper of Canada, a NATO ally. The president has also spoken with Cameron, Hollande and Australian Prime Minister Kevin Rudd in recent days.
Officials said the international community was 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.
The most likely military response would involve sea-launched cruise missile attacks on Syrian military targets. The White House is also studying legal justifications for taking such steps without approval from the United Nations, where Russia is certain to block action at the Security Council.
Italy, meanwhile, is insisting that any strike should be authorized by the Security Council.
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 that 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."
Obama and Secretary of State John Kerry were also making the case for action to counterparts around the world. While the president has not spoken publicly about the deepening crisis this week, Kerry on Monday issued a blistering rebuke of the Assad government's actions.
"By any standard, it is inexcusable and - despite the excuses and equivocations that some have manufactured - it is undeniable," he told reporters at the State Department on Monday.
Assad, who has denied using chemical weapons, 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."
The Syrian leader warned that if the U.S. attacks Syria, it will face "what it has been confronted with in every war since Vietnam: failure."
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 the U.N. 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.
However, some lawmakers from both parties were calling on the president to consult Congress before moving forward. Republican Rep. Scott Rigell of Virginia is asking colleagues to sign a letter to the president urging him to reconvene Congress and seek approval for any military action.
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 probably 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 Syria'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 strikes would probably come during the night and target key military sites.
AP writer Lolita C. Baldor in Washington and AP National Security Writer Robert Burns contributed to this report from Bander Seri Begawan, Brunei.