President Trump’s embattled national security adviser Michael Flynn resigned Monday night, Fox News reported.

Flynn, in his resignation letter, wrote that he "inadvertently" briefed top officials with incomplete information regarding his call with the Russian ambassador.

"I have sincerely apologized to the president and the vice president, and they have accepted my apology," the letter read.

Trump has named Lt. Gen Joseph Keith Kellogg, Jr., as his acting national security adviser.

The White House earlier said Trump was "evaluating the situation" regarding Flynn's conversations with Russia's ambassador to the U.S., deepening the uncertainty surrounding Flynn's future in the new administration.

Flynn apologized privately for the controversy to Vice President Mike Pence, according to a White House official. Pence, relying on information from Flynn, publicly vouched that the retired Army lieutenant general did not discuss U.S. sanctions against Russia in calls late last year.

Flynn has since told the White House that sanctions may have come up.

White House spokesman Sean Spicer said Trump was consulting with Pence 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."

Trump, who comments on a steady stream of issues on his Twitter feed, has been conspicuously silent about the matter since The Washington Post reported last week that Flynn had discussed sanctions with the Russian envoy.

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.

Earlier Monday, White House counselor Kellyanne Conway said Trump had "full confidence" in Flynn, though her assertions were not backed up by other senior Trump aides. Spicer would say only that Flynn was continuing to carry out "his daily functions."

The conflicting signals created confusion at the White House, with reporters crowding around Spicer's office for hours Monday afternoon seeking information on Flynn's future.

(Fox News)