Michael Flynn arriving before a joint news conference between Canadian Prime Minister Justin Trudeau and US President Donald Trump at the White House on Monday. REUTERS/Carlos Barria Michael Flynn abruptly resigned as national security adviser on Monday evening amid an uproar over conversations he had with a Russian diplomat and his broader dealings with Russia.

President Donald Trump named retired Lt. Gen. Joseph Keith Kellogg the acting national security adviser.

The news came soon after a Washington Post report suggested some US officials believed Flynn could be susceptible to Russian blackmail and after he faced new scrutiny over a call with Russia's ambassador to the US during which the pair apparently discussed US sanctions.

The report from The Washington Post on Monday night said Sally Yates, the acting attorney general at the time, in January warned the White House that Flynn had misled officials about his correspondence with Sergey Kislyak, Russia's ambassador to the US.

Trump fired Yates at the end of January after she refused to enforce his now-suspended travel ban.

The Post cited an anonymous official as saying that both the director of national intelligence and the CIA director at the time, James Clapper and John Brennan, expressed concern that "Flynn had put himself in a compromising position" with Russia and agreed that Vice President Mike Pence deserved to know he had been misled. It remains unclear what White House counsel Donald McGahn did with that information.

Flynn said he apologized to Pence on Monday, saying he may have discussed the Obama administration's sanctions with Kislyak before Trump was sworn in, which would've been a breach of protocol. Flynn had previously contended that US sanctions never came up during his conversations with Kislyak.

"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 wrote in his resignation letter Monday night. "I have sincerely apologized to the president and vice president, and they have accepted my apology."

President Barack Obama announced new sanctions on Russia in late December for that country's alleged role in election-related cyberattacks.

Sally Yates, then the US deputy attorney general, at a press conference at the Department of Justice on June 28. Pete Marovich/Getty Images

Major fallout

As the evidence of Flynn's discussions mounted, including transcripts of calls, a spokesman said Flynn "indicated that while he had no recollection of discussing sanctions, he couldn't be certain that the topic never came up."

The damage had already been done, however, as The New York Times reports that three administration officials familiar with the situation reportedly said Pence, who made several television appearances to defend Flynn, was incensed at the national security adviser for apparently withholding information.

Earlier Monday, the White House press secretary, Sean Spicer, said in a statement that the White House was "evaluating the situation" with Flynn. A senior Trump administration official also said officials were aware of the matter and were "working on this for weeks."

Mounting pressure

Both Democratic and Republican lawmakers had urged Flynn to resign. On Monday, Republican Rep. Mike Coffman of Colorado released this statement:

"As national security advisor, Michael Flynn is responsible to the President, Vice President, and the American people. It is his duty to be fully transparent and forthright in his actions — anything less is unacceptable. If in fact he purposely misled the President, he should step down immediately."

Rumors of Flynn's removal were also fueled when sources close to the White House told Politico on Monday that Trump's senior adviser and son-in-law, Jared Kushner, was taking part in the search for potential replacements for Flynn. One of these replacements could be retired Gen. David Petraeus, according to Politico, which said he was scheduled to meet with Trump this week.

"They are trying to figure out the solution to Flynn right now," one of Politico's sources said. "The problem is they don't have it yet. They need to get a solution."

Here's the full text of Flynn's resignation letter: