Donald Trump's national security advisor Michael Flynn resigned amid controversy over his contacts with the Russian government, a stunning first departure from the new president's inner circle less than a month after his inauguration, AFP reports.

The White House said Trump had accepted Flynn's resignation amid allegations the retired three star general discussed US sanctions strategy with Russia's ambassador Sergey Kislyak before taking office.

Flynn -- who once headed US military intelligence -- insisted he was honored to have served the American people in such a "distinguished" manner.

But he admitted that he "inadvertently briefed" the now Vice President Mike Pence with "incomplete information" about his calls with Kislyak.

Pence had publicly defended Flynn, saying he did not discuss sanctions, putting his own credibility into question.

"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," read Flynn's letter, a copy of which was released by the White House.

The White House said Trump has named retired lieutenant general Joseph Kellogg, who was serving as a director on the Joint Chiefs of Staff, to be interim national security advisor.

Flynn's resignation so early in an American administration is unprecedented, and comes after details of his calls with the Russian diplomat were made public -- upping the pressure on Trump to take action.

Several US media outlets on Monday, February 13, reported that top Trump advisors were warned about Flynn's contacts with the Russians early this year.

Questions will now be raised about who knew about the calls and why Trump did not move earlier to replace Flynn.