U.S. intelligence agencies are preparing a report laying out the evidence against Assad's government in chemical weapons attacks on civilians. The classified version would be sent to key members of Congress and a declassified version would be released publicly. The White House says it's already convinced, however, and is planning a possible military response while rounding up support from international partners.
"If there is action taken, it must be clearly defined what the objective is and why" and based on "clear facts," said one of the senior officials, speaking on condition of anonymity because he wasn't authorized to discuss internal deliberations publicly.
The official said the administration is considering more than a single set of military strikes and "the options are not limited just to one day" of assault.
In broad terms, the U.S. and international objective of striking Syria would be to damage the Syrian government's military and weapons to make it difficult to conduct more chemical attacks, and to make Assad think twice about using such weapons in the future.
The manner and timing of Syria's response are among the so-called "next day" questions that the administration is still thinking through as it prepares a possible military action. No additional U.S. defensive weapons have been deployed in the region in anticipation of Syria reprisals, the official said. The U.S. already has Patriot anti-missile batteries in Jordan and Turkey.
The other U.S. official said the administration has determined it can contain any potential Syrian military response in the event that President Barack Obama orders a U.S. attack, which likely would be led by low-flying cruise missiles fired from any of four U.S. Navy destroyers off Syria's coast.
Both officials were granted anonymity in order to discuss internal deliberations on complex questions that surround crafting a response to the Aug. 21 attack in which hundreds of Syrian civilians were killed.
The administration in recent days has made clear it believes it must take punitive action against Syria for its alleged use of chemical weapons, which are banned by international convention. But the senior officials' comments Wednesday made clear that questions about using military force in this circumstance are still being worked out.
The officials said diplomatic and legal issues also are still being discussed internally.
"If any action is taken it will not be taken until all these pieces are in place: the legal issues, the international piece, the consequences thought through, the facts and everything that needs to be tied together," the first senior official said.
The official did not go into detail. Questions may include to what degree military strikes would prevent Assad from using poison gas in the future, and how to respond if he does.
The administration also is concerned that if Assad is not punished, dictatorial leaders of other nations in possession of chemical weapons, like North Korea, might see the failure to act as a sign that they, too, could get away with using the weapons.
Administration officials have said 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 have they yet presented concrete proof that Assad was responsible.
Assad has denied using chemical weapons, calling the allegations "preposterous."
"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 its 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.
The prime minister's office said Wednesday that it will put forward a resolution to the U.N. Security Council condemning the Syrian government for the alleged chemical attack.
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.
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.
"There's no doubt who is responsible for this heinous use of chemical weapons in Syria: the Syrian regime," Biden said.
Ahead of any strike, the U.S. also planned to release additional intelligence it said would directly link Assad to the 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.
Even before releasing that information, U.S. officials said 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.