WASHINGTON (AP) -- The Obama administration tried to bolster its case Tuesday for possible military action against Syria within days, with intelligence agencies preparing to release intercepted communications aimed at proving Bashar Assad perpetrated a large-scale chemical weapons attack on civilians. "There's no doubt who is responsible for this heinous use of chemical weapons in Syria: the Syrian regime," Vice President Joe Biden said.
The U.S. and international partners were unlikely to undertake military action 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 alleged chemical weapons attack.
President Barack Obama and Cameron conferred on response plans Tuesday, their second known conversation in recent days.
Administration officials argued that Assad's actions posed a direct threat to U.S. national security, providing Obama with a potential legal justification for launching a strike without authorization from the United Nations or Congress. However, officials did not detail how the U.S. was directly threatened by an attack contained within Syria's borders. Nor did they present concrete proof that Assad was responsible.
"Allowing the use of chemical weapons on a significant scale to take place without a response would present a significant challenge to, threat to the United States' national security," White House spokesman Jay Carney said.
Assad has denied using chemical weapons, calling the allegations "preposterous."
Obama is weighing a response focused narrowly on punishing Assad for violating international agreements that ban the use of chemical weapons, an act the president repeatedly has said would cross a "red line." Officials said the goal was not to drive the Syrian leader from power or impact the broader trajectory of Syria's bloody civil war, which is now in its third year.
"The options we are considering are not about regime change," Carney told reporters.
According to U.S. officials, the most likely operation would be largely sea-based, with the strikes coming primarily from Navy warships in the Mediterranean Sea. Fighter jets often are deployed to monitor the area and protect the ships, but Syria's robust air defense system makes air strikes more difficult and risky.
Defense Secretary Chuck Hagel said military forces stand ready to strike Syria immediately if the commander in chief gives the order. The Navy has four destroyers in the eastern Mediterranean within range of targets inside Syria and also has warplanes in the region.
"We are ready to go," Hagel said during a television interview while traveling in Asia.
Ahead of any strike, the U.S. also plans to release additional intelligence it says will directly link Assad to the Aug. 21 attack in the Damascus suburbs. Syrian activists say hundreds of people were killed in the attack. A U.S. official said the intelligence report is expected to include "signals intelligence" -- information gathered from intercepted communications.
All of the officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.
Even before releasing that information, U.S. officials said they had very little doubt that Assad was culpable in the attack based on witness reports, information on the number of victims and the symptoms of those killed or injured, and intelligence showing the Syrian government has not lost control of its chemical weapons stockpiles.
Other administration officials echoed Biden's comments, which marked a subtle shift in the administration's rhetoric on who bears responsibility for the attack. Earlier in the week officials would say only that there was "very little doubt" Assad was responsible.
Obama, Biden and other senior administration officials have spent much of the week seeking to rally international support for an aggressive response to the chemical weapons attack. The president spoke Tuesday with Prime Minister Stephen Harper of Canada, a NATO ally, and has also talked to French President Francois Hollande and Australian Prime Minister Kevin Rudd.
Along with Britain, France appears poised to back the U.S. response. In Paris, Hollande said Tuesday that France was "ready to punish those who took the heinous decision to gas innocents." 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.
Italy, meanwhile, was insisting that any strike should be authorized by the U.N. Security Council.
The flurry of action was in stark contrast to Obama's previously restrained approach to Syria's civil war, which has left more than 100,000 people dead, according to U.N. estimates. He has resisted calls for a more robust U.S. response, underscoring the scant appetite among the American public for a long involvement in another Middle East war.