President Donald Trump's embattled national security adviser Michael Flynn has resigned following reports 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 held numerous calls with the Russian ambassador to the US during the transition and gave "incomplete information" about those discussions to Vice President Mike Pence.
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 on national security issues during the campaign.
Flynn apologised to Pence last week, following a Washington Post report asserting that the national security adviser has indeed discussed sanctions with the Russian envoy.
White House spokesman Sean Spicer said Trump was consulting with Pence on Monday about his conversations with the national security adviser. Asked whether the president had been aware that Flynn might discuss sanctions with the Russian envoy, Spicer said, "No, absolutely not."
Earlier in the day White House adviser Kellyanne Conway said that Flynn enjoyed the "full confidence" of President Donald Trump.
A US 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.
Flynn's discussions with the Russian raised questions about whether Flynn 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.
Flynn's conversations also raise questions about Trump's friendly posture toward Russia after US intelligence agencies concluded that Moscow hacked Democratic emails during the election.
Flynn's resignation comes as Trump and his top advisers seek to steady the White House after a rocky start.
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.