WASHINGTON — White House national security adviser Michael Flynn resigned late Monday night amid reports he discussed U.S.-imposed sanctions with a Russian ambassador before President Donald Trump took office.

Earlier Monday, a top aide said Trump was “evaluating the situation” while another described the president as having “full confidence” in Flynn.

Flynn is believed to have discussed sanctions with Sergey Kislyak, Russian ambassador the United States, before Trump’s Jan. 20 inauguration and misled Vice President Mike Pence on the contents of the call.

In a letter announcing his resignation, Flynn said he “inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador. I have sincerely apologized to the President and the Vice President, and they have accepted my apology.”

Trump accepted Flynn’s resignation letter and appointed Keith Kellogg, a decorated retired Army lieutenant general, as acting national security adviser.

Kellogg is one of three candidates Trump is considering as a permanent replacement for Flynn, according to a senior White House official. The other two are David Petraeus, a former CIA director and retired general, and Vice Adm. Robert Harward, a former deputy commander of the U.S. Central Command.

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The White House did not comment publicly on Flynn’s resignation or his permanent replacement.

Flynn’s departure capped a decorated career in public service for the retired lieutenant general and intelligence official, but also marked a setback for an administration just over three weeks old. The subject that fueled Flynn’s exit — the new administration’s connections to Russia — has for months given pause to both Democrats and Republicans.

Monday night, The Washington Post reported that the acting attorney general had informed the Trump White House late last month that she believed Flynn had misled senior administration officials and warned that the national security adviser was potentially vulnerable to Russian blackmail.

The message, delivered by Sally Q. Yates and a senior career national security official to the White House counsel, was prompted by concerns that Flynn, when asked about his calls and texts with the Russian diplomat, had told Pence and others that he had not discussed the Obama administration sanctions on Russia for its interference in the 2016 election, the Post reported, citing current and former U.S. officials. It is unclear what the White House counsel, Donald McGahn, did with the information.

The Post, citing nine sources, reported Thursday that Flynn may have signaled to Russia that it can expect a reprieve from the sanctions.

The Kremlin denied that Flynn and Kislyak discussed the lifting of sanctions, Reuters reported.

Senate Minority Leader Chuck Schumer (D-N.Y.) and House Minority Leader Nancy Pelosi (D-Calif.) separately said that Flynn should be fired if he talked about sanctions with Russia.

Schumer and Sen. Kirsten Gillibrand (D-N.Y.) were not available for comment early Tuesday.

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Flynn sat in the front row Monday at a news conference Trump hosted with Canadian Prime Minister Justin Trudeau.

Trump and Trudeau downplayed their ideological differences on immigration and free trade while pledging to strengthen their relationship.

Trump stressed the need for partnership against terrorism. “In these dangerous times, it is more important than ever that we continue to strengthen our vital alliance,” he said. “The United States is deeply grateful for Canada’s contribution to the counter-ISIS effort.”

Trudeau noted the neighboring countries benefit from more than $2 billion in two-way trade and said in French that “maintaining strong economic ties is vital to our mutual success.”

Trump has threatened to scrap or renegotiate the North American Free Trade Agreement, saying he can get a better deal for the United States.

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He moderated his tone Monday, saying he will be “tweaking” NAFTA, and the impact on Canada will be “much less severe” than on Mexico.

Trump also was asked whether he believes the U.S.-Canadian border is secure. “You can never be totally confident,” he responded.

The president said more generally that he wants the United States to have a “big, beautiful, open door,” but he believes immigrants must be properly vetted to ensure they aren’t a threat.

Trudeau declined to criticize Trump on the travel ban, saying, “The last thing Canadians expect is for me to come down and lecture another country on how they choose to govern themselves.”

With Emily Ngo and Tom Brune