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 in the job for 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. “Unfortunately, because of the fast pace of events, I inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador,” he wrote. “I have sincerely apologized to the president and the vice president, and they have accepted my apology.”

“I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way,” Mr. Flynn wrote.

On Tuesday morning, Mr. Flynn wrote on Twitter: “While I accept full responsibility for my actions, I feel it is unfair that I have been made the sole scapegoat for what happened. But if a scapegoat is what’s needed for this Administration to continue to take this great nation forward, I am proud to do my duty.”

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.

Mr. Flynn was an early and ardent supporter of Mr. Trump’s candidacy, and in his resignation he sought to praise the president. “In just three weeks,” Mr. Flynn said, the new president “has reoriented American foreign policy in fundamental ways to restore America’s leadership position in the world.”

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Michael Flynn’s Resignation Letter

Michael T. Flynn, under scrutiny for his communication with Russia, resigned as President Trump's national security adviser late Monday.

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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.

Earlier Monday, Sean Spicer, the White House press secretary, told reporters that “the president is evaluating the situation” about Mr. Flynn’s future. By Monday evening, Mr. Flynn’s fortunes were rapidly shifting — his resignation came roughly seven hours after Kellyanne Conway, a counselor to the president, said on MSNBC that Mr. Trump had “full confidence” in the retired general.

And when he did step down, it happened so quickly that his resignation does not appear to have been communicated to National Security Council staff members, two of whom said they learned about it from news reports.

Officials said Mr. Pence had told others in the White House that he believed Mr. Flynn lied to him by saying he had not discussed the topic of sanctions on a call with the Russian ambassador in late December. Even the mere discussion of policy — and the apparent attempt to assuage the concerns of an American adversary before Mr. Trump took office — represented a remarkable breach of protocol.

The F.B.I. had been examining Mr. Flynn’s phone calls as he came under growing questions about his interactions with Russian officials and his management of the National Security Council. The blackmail risk envisioned by the Justice Department would have stemmed directly from Mr. Flynn’s attempt to cover his tracks with his bosses. The Russians knew what had been said on the call; thus, if they wanted Mr. Flynn to do something, they could have threatened to expose the lie if he refused.

The Justice Department’s warning to the White House was first reported on Monday night by The Washington Post.

In addition, the Army has been investigating whether Mr. Flynn received money from the Russian government during a trip he took to Moscow in 2015, according to two defense officials. Such a payment might violate the Emoluments Clause of the Constitution, which prohibits former military officers from receiving money from a foreign government without consent from Congress. The defense officials said there was no record that Mr. Flynn, a retired three-star Army general, filed the required paperwork for the trip.

Representative Adam B. Schiff of California, the top Democrat on the House Intelligence Committee, said in a statement late Monday that Mr. Flynn’s resignation would not close the question of his contact with Russian officials.

“General Flynn’s decision to step down as national security adviser was all but ordained the day he misled the country about his secret talks with the Russian ambassador,” said Mr. Schiff, noting that the matter is still under investigation by the House committee.

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Two other Democratic lawmakers — Representative John Conyers Jr. of Michigan and Representative Elijah E. Cummings of Maryland — called for an immediate briefing by the Justice Department and the F.B.I. over the “alarming new disclosures” that Mr. Flynn was a blackmail risk. “We need to know who else within the White House is a current and ongoing risk to our national security,” they said in a statement.

Representative Devin Nunes, Republican of California and the chairman of the House intelligence committee, was supportive of Mr. Flynn until the end. “Washington, D.C., can be a rough town for honorable people, and Flynn — who has always been a soldier, not a politician — deserves America’s gratitude and respect,” Mr. Nunes said in a statement.

The White House had examined a transcript of a wiretapped conversation that Mr. Flynn had with Mr. Kislyak in December, according to administration officials. Mr. Flynn originally told Mr. Pence and others that the call was limited to small talk and holiday pleasantries.