On the same day Obama talked with Canadian Prime Minister Stephen Harper and British Prime Minister David Cameron, Secretary of Defense Chuck Hagel and Secretary of State John Kerry consulted allies and indicated potentially imminent action by a coalition likely to include key NATO partners and regional powers.
Cameron is proposing a U.N. Security Council resolution "condemning the chemical weapons attack by Assad and authorizing necessary measures to protect civilians," but certain opposition by Syrian ally Russia and possibly by China doomed its chances.
Instead, a limited coalition of NATO partners and some Arab League members appeared more likely to provide political backing for Obama to order U.S. missile strikes. An Arab League spokesman condemned the al-Assad regime on Tuesday for the chemical attack.
In another move, the United States postponed its involvement in talks scheduled for this week in Geneva on seeking a political solution to the Syrian civil war. Russia expressed disappointment at the U.S. decision and warned against any Western military strike on Syria, as did Iran.
The United States has already moved warships armed with cruise missiles into the region, and a U.S. official said Wednesday that two Navy submarines also were in the eastern Mediterranean, though it was unclear whether they would be involved in any military action.
Hagel told the BBC on Tuesday that forces were ready to carry out a strike if ordered. A senior Defense Department official told CNN that any strike could be completed "within several days."
"We are ready to go, like that," said Hagel, adding that "the options are there, the United States Department of Defense is ready to carry out those options."
For almost two years, Obama has avoided direct military involvement in Syria's civil war, only escalating aid to rebel fighters in June after suspected smaller-scale chemical weapons attacks by Syrian government forces.
However, last week's attack obliterated the "red line" Obama set just over a year ago against the use of Syria's chemical weapons stocks.
Vice President Joe Biden made clear that the administration's view of who was to blame for last week's event, telling the American Legion on Tuesday that "there is no doubt who is responsible for the heinous use of chemical weapons -- the Syrian regime."
The White House has ruled out sending ground troops to Syria or implementing a no-fly zone to blunt al-Assad's aerial superiority over rebels fighting to oust his regime.
A first step to any U.S. action will be the public release of a declassified U.S. intelligence report on the chemical weapons that rebels claim killed 1,300 people. Carney said the report was expected to come out this week.
It would set in motion the process that could lead to missile strikes or other responses, depending on the administration's ability to line up international support and prevent any domestic obstacles.
Legislator: Congress can't 'be pushed aside'
Administration officials will brief some senators on Syria via conference calls Thursday, several sources said.
More than 90 members of Congress, most of them Republican, have signed a letter to the president urging him "to consult and receive authorization" before authorizing any such military action, according to the office of GOP Rep. Scott Rigell of Virginia.
"I appreciate and respect that the president is engaging members of Congress," Rigell said Wednesday in reference to outreach by administration officials to legislative leaders and others. "This is good, and I encourage more of it, both on the Senate side and the House. But it is not in any respect a substitute for formally calling us into session, a joint session, laying the facts before us without disclosing of course sources and methods of intelligence. Then we, as the representatives of the American people, can weigh in on this as we should."
However, Democratic Rep. Adam Smith of Washington said that "the historical precedence is no, that the president doesn't require congressional action for a variety of different things."
Some Republican senators also noted that the War Powers Act allows Obama to order limited missile strikes and report back to Congress afterward.
Last month, Joint Chiefs Chairman Gen. Martin Dempsey provided Congress with a list of declassified U.S. military options for Syria that emphasized the high costs and risks of what he said would amount to "an act of war" at a time of deep budget cuts.