WASHINGTON — President Donald Trump’s embattled national security adviser Michael Flynn resigned following reports he misled Vice President Mike Pence and other officials about his contacts with Russia. His departure upends Trump’s senior team after less than a 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.