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.
Even after the latest use of chemical weapons, the president has ruled out putting American troops on the ground in Syria and officials said they were not considering setting up a unilateral no-fly zone.
Instead, officials said it was likely missiles could be used to target weapons arsenals, command and control centers, radar and communications facilities, and other military headquarters. Less likely was a strike on a chemical weapons site because of the risk of releasing toxic gases.
Military experts and U.S. officials said the strikes probably would come during the night and target key military sites.
The Obama administration's desire to respond quickly to last week's attack likely puts the president in the position of taking military action without formal approval from the United Nations. Russia, which has helped prop up Assad throughout the civil war, is certain to block U.S. attempts to seek a resolution approving force at the U.N. Security Council.
It's unclear whether the president will seek some type of authorization from Congress, which is out of session until Sept. 9. Rep. Scott Rigell, R-Va., is asking colleagues to sign a letter to Obama that urges him to reconvene Congress and seek approval for any military action.
The 1973 War Powers Resolution reaffirmed Congress' constitutional responsibility to declare war and put a 60-day time limit on the president's ability to take unauthorized, emergency military action. Since then, commanders in chief of both political parties have maintained that the resolution is unconstitutional and have regularly disregarded it.
When the U.S. acted with allies against Libyan dictator Moammar Gadhafi two years ago, Obama maintained military operations for more than three months without congressional authorization. He said the U.S. wasn't violating the War Powers Resolution because Americans were supporting a NATO-led operation and weren't engaged in full-blown hostilities.
Burns reported from Bander Seri Begawan, Brunei. Associated Press writers Lolita C. Baldor, Matthew Lee and Bradley Klapper contributed to this report.
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