Michael Flynn with Donald Trump in Palm Beach, Florida, on December 21. REUTERS/Carlos Barria President Donald Trump has known "for weeks" that Michael Flynn discussed US sanctions on Russia during his calls with Russia's ambassador to the US, a senior White House official told The Washington Post on Monday.

"We've been working on this for weeks," a senior Trump administration official told The Post when asked whether the White House was aware of reports that Flynn had discussed the sanctions.

The White House was apparently briefed by Sally Yates, then the acting attorney general, on January 23 about content of Flynn's calls with Ambassador Sergey Kislyak that Yates considered "highly significant" and "potentially illegal."

The Post said intelligence officials began looking into potential contact between Trump's transition team and Russian officials in December when Russian President Vladimir Putin decided not to retaliate against sanctions introduced by the Obama administration. Officials discovered that Flynn called Kislyak the day President Barack Obama imposed the new penalties and gave him "the impression that the sanctions would be revisited at a later time."

The Post report said FBI Director James Comey wanted to wait to brief the new administration on the calls until after the investigation into Flynn and his contact with Russian officials was completed but changed his mind on January 15, after he saw Vice President Mike Pence defend Flynn in an interview with CBS. Pence wrongly claimed in the interview that sanctions had not been discussed during calls between Flynn and Kislyak.

Flynn resigned Monday night, saying in his resignation letter that "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."

"In the end, it was misleading the vice president that made the situation unsustainable," Kellyanne Conway, the senior counselor to Trump, told NBC on Tuesday morning. She had said Monday afternoon that Flynn still enjoyed the "full confidence" of the president before the White House subsequently walked back on that statement.

Indeed, Pence was reportedly "incensed" at Flynn for misleading him about the calls. But he was also apparently briefed by Yates about those calls just over a week after he went on CBS.

Yates, backed by the Department of Justice, the FBI, and the Office of the Director of National Intelligence, reportedly went as far as to warn the White House that Flynn could be blackmailed by the Kremlin, which knew about the secret conversations.

The pressure did not start building on Flynn to resign, however, until last Thursday, when The Washington Post and The New York Times reported, citing nearly two dozen current and former officials, that Flynn had discussed sensitive national security information with Kislyak while Flynn was still a private citizen.

When asked about those reports the next day, Trump replied that he was "not aware" of them and that he would "look into" it.

"Here's what's bothering me," Rep. Adam Schiff, the top Democrat on the House intelligence committee, tweeted on Tuesday. "Flynn's lie about Russia didn't bring him down. Exposure of lie did. [White House] knew weeks ago & did nothing. Why?"

When asked that same question by ABC's George Stephanopoulos on Tuesday morning, Conway replied that Stephanopoulos was "presuming all of the information" in the Washington Post article was "completely factual."

"I don't know all the details," Conway added. "I'm not here to say who knew what when."

Conway had said Monday afternoon that Flynn still enjoyed the "full confidence" of the president. The White House then walked that statement back, saying that Trump was "still evaluating" the situation."