WASHINGTON, Feb.13 (Xinhua) -- U.S. National Security Adviser Michael Flynn resigned Monday night after days of reports over whether he discussed American sanctions against Russia in a phone call with a Russian diplomat before President Donald Trump's inauguration.

Flynn has been the first senior government official to have left the Trump administration since the new leader taking office in January.

Retired Army Gen. Keith Kellogg, a top policy adviser for Trump's presidential campaign, was appointed acting national security adviser, according to local media reports.

Earlier on Monday, it was revealed that the Justice Department had informed the White House that Flynn was not fully forthright about his conversations with the Russian ambassador to the United States and could be vulnerable to blackmail by Moscow.

Flynn's status was considered perilous after it was disclosed that he had misled Vice President Mike Pence about his communications with the ambassador, local media reported.