WASHINGTON — National security adviser Michael Flynn has resigned following reports he misled Vice President Mike Pence about contacts with a Russian diplomat, up-ending President Donald Trump’s White House team less than a month after his inauguration.

In a resignation letter, Flynn said he gave Pence and others “incomplete information” about his calls with Russia’s ambassador to the U.S. The vice president, apparently relying on information from Flynn, initially said the national security adviser had not discussed sanctions with the Russian envoy, though Flynn later conceded the issue may have come up.

Such conversations would breach diplomatic protocol and possibly violate the Logan Act, a law aimed at keeping private citizens from conducting U.S. diplomacy. The Justice Department also had warned the White House late last month that Flynn could be in a compromised position because of contradictions between his public depictions of the calls and what intelligence officials knew to be true based on routine recordings of communications with foreign officials who are in the U.S.

Kellyanne Conway, a close aide to Trump, had said Monday that Flynn continued to have the “full confidence” of the president. Today, she said in televised interviews that Trump had supported Flynn out of loyalty but that the situation reached a “fever pitch” and had become “unsustainable.”

“By night’s end, Mike Flynn had decided it was best to resign. He knew he’d become a lightning rod, and he made that decision,” Conway told NBC’s “Today” show.

When asked why the White House didn’t move sooner after being warned by the Justice Department that Flynn was at risk of blackmail, Conway was vague: “As time wore on, obviously the situation became unsustainable,” she repeated. She added: “We’re moving on.”

Trump named retired Lt. Gen. Keith Kellogg as the acting national security adviser. Kellogg had previously been appointed the National Security Council chief of staff and advised Trump during the campaign. Trump is also considering former CIA Director David Petraeus and Vice Admiral Robert Harward, a U.S. Navy SEAL, for the post, according to a senior administration official.

A U.S. official told The Associated Press that Flynn was in frequent contact with Ambassador Sergey Kislyak on the day the Obama administration slapped sanctions on Russia for election-related hacking, as well as at other times during the transition.

An administration official and two people with knowledge of the situation confirmed the Justice Department warnings on condition of anonymity because they were not authorized to discuss the matter publicly. It was unclear when Trump and Pence learned about the Justice Department outreach.

The Washington Post was the first to report the communication between former acting attorney general Sally Yates, a holdover from the Obama administration, and the Trump White House. The Post also first reported last week that Flynn had indeed spoken about sanctions with the Russian ambassador.

Trump never voiced public support for Flynn after that initial report but continued to keep his national security adviser close.

The White House officials sent contradictory messages, meantime, about Flynn’s job status. While Conway was remarking that Trump had “full confidence” in the retired general, White House press secretary Sean Spicer said the president was “evaluating the situation” and consulting with Pence about his conversations with the national security adviser.