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Michael T. Flynn, the national security adviser, resigned on Monday night after it was revealed that he had misled Vice President Mike Pence and other top White House officials about his conversations with the Russian ambassador to the United States.

Mr. Flynn, who served less than a month, said he had given “incomplete information” regarding a telephone call he had with the ambassador in late December about American sanctions against Russia, weeks before President Trump’s inauguration. Mr. Flynn previously had denied that he had any substantive conversations with Ambassador Sergey I. Kislyak, and Mr. Pence repeated that claim in television interviews as recently as this month.

But on Monday, a former administration official said the Justice Department warned the White House last month that Mr. Flynn had not been fully forthright about his conversations with the ambassador. As a result, the Justice Department feared that Mr. Flynn could be vulnerable to blackmail by Moscow.

In his resignation letter, which the White House emailed to reporters, Mr. Flynn said he had held numerous calls with foreign officials during the transition.  Mr. Flynn wrote. that he is  tendering his resignation, and he was honoured to have served the nation and the American people in such a distinguished way.

 The White House said in the statement that it was replacing Mr. Flynn with retired Lt. Gen. Joseph K. Kellogg Jr. of the army, a Vietnam War veteran, as acting national security adviser.

But in doing so, he inadvertently illustrated the brevity of his tumultuous run at the National Security Council, and the chaos that has gripped the White House in the first weeks of the Trump administration — and created a sense of uncertainty around the world.