Administration officials also said Assad's actions posed a direct threat to U.S. national security, providing President Barack 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 Tuesday.
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 were expected to vote on a motion clearing the way for a British response.
Obama and Cameron spoke Tuesday, their second known conversation since the weekend. A Cameron spokesman said the two leaders agreed that a chemical attack had taken place, and that the Assad regime was responsible. Cameron "confirmed that the government had not yet taken a decision on the specific nature of our response, but that it would be legal and specific to the chemical weapons attack," the spokesman said.
Also Tuesday, Vice President Joe Biden became the highest-ranking U.S. official to charge that Assad's government fired chemical weapons last week near Damascus. Assad has denied using chemical weapons, calling the allegations "preposterous."
"There's no doubt who is responsible for this heinous use of chemical weapons in Syria: the Syrian regime," Biden said.
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 nor affect 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 military 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 airstrikes more difficult and risky.
Defense Secretary Chuck Hagel said military forces stood ready to strike Syria immediately if the commander in chief gave 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 in a BBC television interview Tuesday while traveling in Asia.
Ahead of any strike, the U.S. also planned to release additional intelligence it said would directly link Assad to the Aug. 21 attack in the Damascus suburbs. Syrian activists said hundreds of people were killed in the attack. A U.S. official said the intelligence report was 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.
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