Michael Flynn before the start of the joint news conference between President Donald Trump and Japanese Prime Minister Shinzo Abe in the East Room of the White House on Friday. Carolyn Kaster/AP President Donald Trump's senior counselor, Kellyanne Conway, told the "Today" show on Tuesday morning that Michael Flynn's role as national security adviser became "unsustainable" after it became clear that he "misled" the vice president.

"In the end, it was misleading the vice president that made the situation unsustainable," Conway said, adding that Flynn's "inability to remember" the content of his conversations with Russia's ambassador to the US, Sergey Kislyak, contributed to the fallout.

Flynn resigned four days after The Washington Post and The New York Times reported, together citing nearly a dozen current and former officials, that he had spoken with the Russian ambassador about sanctions before Trump was sworn in, despite Flynn's repeated denials that anything sensitive was discussed on the calls.

Pence was reportedly "incensed" at Flynn for misleading him about the calls, The New York Times reported on Monday. The vice president had defended Flynn in an interview with CBS on January 15, saying Flynn "didn't discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia."

"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."

Trump with Chief of Staff Reince Priebus, Vice President Mike Pence, senior adviser Steve Bannon, communications director Sean Spicer, and Flynn. REUTERS/Jonathan Ernst

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

The White House was apparently briefed on the content of these calls on January 23, when Sally Yates, then the acting attorney general — backed by the Department of Justice, the FBI, and the director of national intelligence — reportedly warned the president that Flynn could be blackmailed by the Kremlin, which knew about the secret conversations.

Yates, who, along with top US intelligence officials, obtained transcripts of Flynn's calls with Kislyak in late December, considered Flynn's comments in at least one of his calls with Kislyak to be "highly significant" and "potentially illegal," The Washington Post reported, citing a US official close to her.

FBI Director James Comey reportedly 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 after he saw the vice president claim, wrongly, on national television that sanctions had not been discussed on the calls.

Flynn with Russian President Vladimir Putin, center right, in Moscow at an exhibition marking the 10th anniversary of the Russia Today 24-hour English-language TV news channel. Mikhail Klimentyev/Sputnik, Kremlin Pool Photo via AP

Conway, a senior counselor to Trump who is among his closest confidantes, continued to claim as late as 4 p.m. ET on Monday that there was no evidence Flynn had purposefully misled the vice president.

When pressed by MSNBC's Steve Kornacki on Monday about whether Trump was bothered by the fact that Flynn may have lied to White House officials about the calls, Conway replied that Kornacki was "asking hypotheticals."

"What if it's not true?" she asked, saying Flynn had merely said he could "not recall" whether sanctions were discussed on the calls.

Conway added that Flynn continued to "enjoy the full confidence of the president" — a claim that was contradicted minutes later by the White House press secretary, Sean Spicer, who told reporters that Trump was still "evaluating the situation" involving Flynn.

On Tuesday morning, asked by ABC's George Stephanopoulos why the White House did not act on information compiled by US intelligence officials that Flynn may have been compromised by Russia, 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."