President Donald Trump's embattled national security adviser Michael Flynn resigned late Monday night, following reports that he had misled Vice President Mike Pence and other officials about his contacts with Russia. His departure upends Trump's senior team after less than one month in office.

In a resignation letter, Flynn said he gave Vice President Mike 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.

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.

The Trump team's account of Flynn's discussions with the Russian envoy changed repeatedly over several weeks, including the number of contacts, the dates of those contacts and ultimately, the content of the conversations.

Late last month, the Justice Department warned the White House that Flynn could be in a compromised position as a result of the contradictions between the public depictions of the calls and what intelligence officials knew to be true based on recordings of the conversations, which were picked up as part of routine monitoring of foreign officials communications in the U.S.

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 the 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. Flynn spent the weekend at Trump's Mar-a-Lago estate and was in the president's daily briefing and calls with foreign leaders Monday. He sat in the front row of Trump's news conference with Canadian Prime Minister Justin Trudeau Monday afternoon.

White House officials sent contradictory messages about Flynn's status throughout the day. Counselor Kellyanne Conway said Trump had "full confidence" in Flynn, while spokesman Sean Spicer said the president was "evaluating the situation" and consulting with Pence about his conversations with the national security adviser.

Asked whether the president had been aware that Flynn might have planned to discuss sanctions with the Russian envoy, Spicer said, "No, absolutely not."

Flynn's discussions with the Russian raised questions about whether he offered assurances about the incoming administration's new approach. Such conversations would breach diplomatic protocol and possibly violate the Logan Act, a law aimed at keeping citizens from conducting diplomacy.

Administration officials said misleading Pence was ultimately Flynn's downfall, though they insisted he resigned and was not fired by Trump.

Flynn was spotted near the Oval Office just after 10 p.m. Monday. As uncertainty about his future swirled, several of the president's top advisers, including chief of staff Reince Priebus and counsel Don McGahn, ducked in and out of late-night meetings in the West Wing.

California Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, said Flynn's resignation "does not end questions over his contacts with the Russians." He said the White House has yet to be forthcoming about whether Flynn was acting at the behest of the president or others.

Flynn's resignation comes as Trump and his top advisers seek to steady the White House after a rocky start. The president, who seeks input from a wide range of business associates, friends and colleagues, has been asking people their opinions on his senior team, including Spicer and chief of staff Reince Priebus.

Advisers have privately conceded that the White House spit out too many disparate messages in the first few weeks, though they also note that the president's own tweets sometimes muddy the day's plans before most of the White House staff has arrived for work.

Trump voiced support for Priebus Monday, saying the chief of staff was doing, "not a good job, a great job." But he did not make a similar show of support for his national security adviser.

Over the weekend, Trump told associates he was troubled by Flynn's situation, but did not say whether he planned to ask him to step down, according to a person who spoke with him recently. Flynn was a loyal Trump supporter during the campaign, but he was viewed skeptically by some in the administration's national security circles, in part because of his ties to Russia.

In 2015, Flynn was paid to attend a gala dinner for Russia Today, a Kremlin-backed television station, and sat next to Russian President Vladimir Putin during the event.

Flynn apologized to Pence about the matter on Friday, according to an administration official. The official said Pence was relying on information from Flynn when he went on television and denied that sanctions were discussed with Kislyak.

Kellogg takes the helm of the National Security Council at a time when the young administration is grappling with a series of national security challenges, including North Korea's reported ballistic missile launch. The president, who was joined at his Mar-a-Lago estate by Japanese Prime Minister Shinzo Abe over the weekend, voiced solidarity with Japan.

The White House is also dealing with fallout from the rocky rollout of Trump's immigration executive order, which has been blocked by the courts. The order was intended to suspend the nation's refugee program and bar citizens of seven Muslim-majority countries from entering the United States.