US National Security Advisor Michael Flynn boards Air Force One at West Palm Beach International airport in West Palm Beach, Florida US, February 12, 2017. [Photo/Agencies]

US National Security Adviser Michael Flynn has resigned, CNN said citing two sources. Flynn is the first senior government official to leave the Trump administration.

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.

"Unfortunately, because of the fast pace of events, I inadvertently briefed the vice president-elect and others with incomplete information 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," Flynn said in his resignation letter.

Retired General Keith Kellogg, who has been the chief of staff at the National Security Council, has been named acting national security adviser, White House official said.

The Justice Department warned the Trump administration weeks ago that embattled national security adviser Michael Flynn's contacts with Russia could leave him in a compromised position, an administration official and two other people with knowledge of the situation told The Associated Press Monday night.

Flynn, a retired US Army lieutenant general, was an early supporter of Trump and shares his interest in shaking up the establishment in Washington. He has frequently raised eyebrows among Washington's foreign policy establishment for trying to persuade Trump to warm up US relations with Russia.