WASHINGTON - As the White House sought to reassure Americans that it has fixed mistakes that nearly allowed al-Qaida to take down a U.S.-bound airliner last Christmas, it acknowledged another security misstep Wednesday:

The nation's top intelligence official was out of the loop about a terror plot and numerous arrests this week in Britain.

Director of National Intelligence James Clapper appeared stumped Tuesday night when asked on ABC News whether a significant terror plot uncovered in London could have security implications in the United States. The plot received wide news coverage this week.

"London?" Clapper asked, looking across the table at Obama's homeland security adviser, John Brennan, who was also being interviewed.

Wednesday, Brennan sought to allay fears about a holiday season terrorist attack during a White House news conference. Asked why Clapper was out of the loop on such a major incident, he said the director was preoccupied with tensions in Korea and a tentative nuclear weapons treaty with Russia.

"Should he have been briefed by his staff on those arrests?" Brennan said. "Yes."

It was an embarrassing moment for Obama and for the embattled position of director of national intelligence. The job, created after the Sept. 11, 2001 attacks, has failed to live up to its billing as a central, strong overseer of the nation's intelligence infrastructure. And it reinforced the impression that Brennan functions as the de facto intelligence director.

The gaffe forced the White House off its message of reassurance to the public. It has been the most challenging year yet for Obama on the terrorism front. After the failed attack last Christmas, the administration faced an attempted car bombing in Times Square, a nearly successful attack on U.S.-bound cargo planes and several nascent plots disrupted by the FBI.

Security officials have been on edge for days because of an increase in intelligence "chatter" about a possible attack.

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