The White House has reportedly known about Flynn's calls with Russian ambassador 'for weeks'

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.

The report said the White House was briefed by acting Attorney General Sally Yates on January 23 about the content of Flynn's calls with Sergey Kislyak, the Russian ambassador to the US, which Yates considered to be "highly significant" and "potentially illegal."

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 Obama's new sanctions, according to the Post. Officials discovered that Flynn called Kislyak the day 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 on the 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, 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 Director of National Intelligence, 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 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."