Assad vowed his country "will defend itself against any aggression."
The White House sought to ease growing concerns on Capitol Hill, deploying a bevy of top administration officials to brief lawmakers Thursday evening on U.S. intelligence assessments. Obama also discussed the situation in Syria with House Speaker John Boehner, R-Ohio, who wrote to the president earlier this week seeking a legal justification for a military strike and the objectives of any potential action.
Yet it appeared likely that an American military operation could happen without formal authorization from Capitol Hill or the United Nations, where Russia has blocked efforts to seek a resolution authorizing the use of force. It remained unclear whether the U.S. would launch strikes on its own or in concert with allies like Britain and France.
Waiting for British participation would mean holding off on a strike at least until the weekend. Prime Minister David Cameron said his country would not join in military efforts until a U.N. chemical weapons inspection team on the ground in Syria releases its findings.
Some of the U.N. chemical weapons experts will travel directly from Syria on Saturday to different laboratories around Europe to deliver "an extensive amount of material" gathered, U.N. spokesman Farhan Haq said. While the mandate of the U.N. team is to determine whether chemical agents were used in the attack, not who was responsible, Haq suggested the evidence — which includes biological samples and witness interviews — might give an indication of who deployed gases.
Obama and other top officials say Assad's government is responsible for the Aug. 21 attack. Yet the administration has not revealed definitive evidence to back its claims. U.S. officials say the intelligence assessments are no "slam dunk," with questions remaining about who actually controls some of Syria's chemical weapons stores and doubts about whether Assad himself ordered the strike.
The officials spoke on condition of anonymity because they were not authorized to discuss the intelligence publicly.
Despite shortcomings in the intelligence, the White House signaled urgency in acting, with spokesman Josh Earnest saying the president believes there is a "compressed timeframe" for responding.
"It is important for the Assad regime and other totalitarian dictators around the world to understand that the international community will not tolerate the indiscriminate, widespread use of chemical weapons, particularly against women and children as they're sleeping in their beds," he said.
But many Congress members were pressing Obama to explain the need for military action and address fears that such a move might draw the U.S. deeper into the Syrian civil war. Both Democrats and Republicans were among lawmakers protesting that Obama hasn't made the case for a military strike, with some arguing that the president needs congressional authorization to order an attack.
The White House has not responded directly to Boehner's letter seeking more answers about Syria operations and the speaker's office appeared unsatisfied after the president's call Thursday.
"Only the president can answer these questions, and it is clear that further dialogue and consultation with Congress, as well as communication with the American public, will be needed," Boehner spokesman Brendan Buck said.
Washington Rep. Adam Smith, senior Democrat on the House Armed Services Committee, cautioned that an attack might be ineffective and might draw the United States into the Syrian civil war, now in its third year.
"Simply lashing out with military force under the banner of 'doing something' will not secure our interests in Syria," Smith said in a statement.
Thursday's briefing for lawmakers was expected to include only unclassified intelligence, meaning that key details that could more clearly link Assad to a chemical attack might not be included. A similar intelligence report is also expected to be released publicly.
The lineup for the teleconference underscored the gravity of the matter. The briefers are Obama's national security adviser and intelligence chief, Susan Rice and James Clapper, alongside Secretary of State John Kerry, Defense Secretary Hagel and Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff.
While Obama declared unequivocally this week that the U.S. had "concluded" that Assad's government was behind the attack, he presented no definitive proof. And in Britain, Cameron said there was still a sliver of uncertainty about who was to blame.
Obama continued making his case for a robust response to world leaders, speaking Thursday with German Chancellor Angela Merkel. With national elections scheduled in Germany for next month, Merkel is unlikely to pull her country into a military conflict.
Merkel also discussed Syria by phone Thursday with Russian President Vladimir Putin, insisting that the attack "requires an international reaction," Merkel spokesman Steffen Seibert said.
France said Thursday that its military is preparing for a possible operation in Syria, though officials stopped short of announcing an armed intervention.
Defense Secretary Chuck Hagel said Thursday that any U.S. action against would be part of "an international collaboration." But he did not expand on what that would entail.
Obama has ruled out putting American forces on the ground in Syria or setting up a no-fly zone over the country. He's also said any U.S. response to the chemical weapons attack would be limited in scope and aimed solely at punishing Assad for deploying deadly gases, not at regime change.
"We do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable," he said during a television interview.
The most likely military option would be Tomahawk cruise missile strikes from four Navy destroyers in the eastern Mediterranean Sea. At a minimum, Western forces are expected to strike targets that symbolize Assad's military and political might: military and national police headquarters, including the Defense Ministry; the Syrian military's general staff; and the four-brigade Republican Guard that is in charge of protecting Damascus, Assad's seat of power. Assad's ruling Baath Party headquarters could be targeted, too.
U.S. officials also are considering attacking military command centers and vital forces, communications hubs and weapons caches, including ballistic missile batteries.
AP writers Kimberly Dozier, Matt Apuzzo, Donna Cassata and Bradley Klapper in Washington, Alexandra Olson at the United Nations, and Robert Burns in Manila, Philippines, contributed to this report.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.