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WASHINGTON--President Obama prepared Thursday for the possibility of launching a U.S. military action against Syria within days, even as Britain opted out in a vote by Parliament.

Advisers said the president would be willing to retaliate against Syria alone.

"The president of the United States is elected with the duty to protect the national security interests in the United States of America," White House spokesman Josh Earnest said.

Caitlin Hayden, National Security Council spokeswoman, said the United States would continue to consult with Britain but Obama would make decisions based on "the best interests of the United States."

Earlier, the White House stepped up efforts to consult with Congress in advance of any U.S. military intervention in Syria, including private communications between President Obama and House Speaker John Boehner, R-Ohio, and a conference call for congressional leaders with senior administration officials.

"That conference call is just the latest in a series of robust congressional consultations that everybody from the president on down in the administration have been engaged in over the last few days," Earnest said of the Thursday evening briefing.

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Officials including National Security Adviser Susan Rice, Secretary of State John Kerry, Secretary of Defense Chuck Hagel, and Director of National Intelligence James Clapper briefed leaders and committee chairman with jurisdiction on foreign policy and the U.S. military.

More than one quarter of the 435-member House has signed a letter authored by Rep. Scott Rigell, R-Va., calling on the president to seek a formal vote for congressional authorization for action in Syria. Boehner has stopped short of calling for a vote, but has publicly chided the president for not making a strong enough case to either Congress or the public.

Following a private call with the president on Thursday, Brendan Buck, a spokesman for Boehner said "it is clear that further dialogue and consultation with Congress, as well as communication with the American public, will be needed."

The White House is unlikely to seek formal congressional approval, but Earnest said the White House is making an effort to consult closely with Congress.

Contributing: The Associated Press

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