President Donald Trump's national security adviser, Michael Flynn, resigned on Monday night in the midst of a raging controversy about the nature of his conversations with a Russian official.

According to media reports, Trump has accepted Flynn’s resignation letter and appointed Keith Kellogg, a retired army lieutenant general, as acting national security adviser.

The Washington Post reported that in his resignation letter to Trump, Flynn defended his conversations with the Russian ambassador to the United States.

“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,” Flynn was reported to have written.

“I have sincerely apologized to the president and the vice president, and they have accepted my apology,” the report said.

The Associated Press reported that 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, reported AP.

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.