WASHINGTON, Feb.14 (Xinhua) -- U.S. National Security Adviser Michael Flynn resigned Monday night after days of reports over whether he discussed American sanctions against Russia in phone calls with a Russian diplomat before President Donald Trump's inauguration.

Flynn is the first senior government official to leave the Trump administration since the new leader took office in January.

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

A possible replacement for Flynn was retired General David Petraeus, also a former CIA director, who was scheduled to meet Trump on Tuesday, according to a Politico report.

Other options for the post include Stephen Hadley, who served as national security adviser to former President George W. Bush; Tom Bossert, who now oversees cyber security under the Trump administration; retired Admiral James Stavridis, a former NATO supreme commander; and John Kelly, head of Department of Homeland Security, said the report.

Flynn spent the weekend with Trump in Florida at the president's Mar-a-Lago resort and was part of the delegation welcoming Canadian Prime Minister Justin Trudeau to the White House earlier on Monday.

Flynn's resignation came amid rampant reports that he lied about improperly discussing U.S. sanctions on Russia by the Obama administration with the Russian ambassador to the United States in December, weeks before Trump's inauguration.

Flynn acknowledged speaking to the ambassador then but denied discussing the sanctions. Vice President Mike Pence was reportedly misled and went on television to back up Flynn.

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 and could be vulnerable to blackmail by Moscow.

Flynn, a retired three-star general, was forced out as director of the Defense Intelligence Agency in 2014. He was one of the first and most active military officers to back Trump during his presidential campaign.