President Donald Trump's ouster of national security adviser Michael Flynn, and the circumstances leading up to it, have quickly become a major crisis for the fledgling administration, forcing the White House on the defensive and precipitating the first significant breach in relations between Trump and an increasingly restive Republican Congress.

Even as the White House described Trump's "immediate, decisive" action in demanding Flynn's resignation late Monday as the end of an unfortunate episode, senior GOP lawmakers were buckling under growing pressure to investigate it.

Senate Majority Leader Mitch McConnell, R-Ky., said Tuesday that it was "highly likely" that the events leading to Flynn's departure would be added to a broader probe into Russian meddling in the U.S. presidential election. Intercepts showed that Flynn discussed U.S. sanctions in a phone call with the Russian ambassador - a conversation topic that Flynn first denied and then said he later could not recall.

McConnell's comments followed White House revelations that Trump was aware "for weeks" that Flynn had misled Vice President Mike Pence and others about the content of his late December talks with Russian Ambassador Sergey Kislyak.

White House counsel Don McGahn told Trump in a briefing late last month that Flynn, despite his claims to the contrary, had discussed U.S. sanctions imposed on Russia by the Obama administration in late December, press secretary Sean Spicer said Tuesday. That briefing, he said, came "immediately" after Sally Yates, then the acting attorney general, informed McGahn on Jan. 26 about discrepancies between intercepts of Kislyak's phone calls and public statements by Pence and others that there had been no discussion of sanctions.

Trump brought in senior strategist Stephen Bannon and White House Chief of Staff Reince Priebus to join the discussion with McGahn, according to two officials familiar with the conversations who spoke on the condition of anonymity because they were not authorized to speak publicly.

McGahn then conferred with Yates again the following day, Jan. 27, to try to glean more information, these two officials said. Within the White House, the matter was viewed skeptically and Trump, Bannon, Priebus and McGahn for several days remained among the few people briefed, they said.

Over the next two weeks, the officials said, Flynn was asked multiple times about what exactly he had said. He brushed aside the suggestion that he had spoken about sanctions with the ambassador - denials that kept him afloat within the White House even as he was being actively evaluated, they said.

It was not until a Washington Post report last Thursday, in which Flynn was quoted as saying that he had no "recollection" of discussing sanctions but couldn't be sure that he hadn't, that the downward slide culminating in Monday's firing began, several administration officials said.

"We've been reviewing and evaluating this issue with respect to General Flynn on a daily basis for a few weeks, trying to ascertain the truth," Spicer said at the daily White House press briefing. He emphasized that an internal White House inquiry had concluded that nothing Flynn discussed with the Russian was illegal, but that he had "broken trust" with Trump by not telling the truth about the talks.

When asked whether Trump told Flynn to talk to Kislyak about sanctions, Spicer responded: "No, absolutely not."

Asked why Trump had waited nearly three weeks to act after what Spicer called a "heads-up" from the Justice Department, he said once the question of legality was settled, "then it became a phase of determining whether or not [Flynn's] action on this and a whole host of other issues undermined" Trump's trust. He declined to specify the "other issues."

In an interview conducted early Monday and published Tuesday by the Daily Caller, Flynn said that he did not specifically discuss sanctions with Kislyak, but rather then-President Barack Obama's simultaneous expulsion of 35 Russian diplomats. He said he told the ambassador that "we'll review everything" following Trump's inauguration.

Current and former U.S. officials have said, however, that much of the conversation was about sanctions, and that Flynn suggested that Moscow not respond in kind to the expulsions - advice that Russian President Vladimir Putin took in declining to take retaliatory action.

Although Trump has not publicly mentioned his view of the sanctions, Spicer said the president "has made it very clear he expects the Russian government to de-escalate violence in the Ukraine and return Crimea," even as he hopes to cooperate with Putin on terrorism.

Asked Tuesday on a flight to Brussels about Flynn's ouster, Defense Secretary Jim Mattis said it "has no impact" on his job. "It doesn't change my message at all, and who is on the president's staff is who I will work with," Mattis said.

Mattis was on his way to a meeting of NATO defense ministers, who were expected to discuss their significant concerns about Russian aggression. During his confirmation hearing, Mattis placed Russia first among threats to U.S. security.

Officials inside the National Security Council described low morale and concern about the future. The "worthless" message at a five-minute staff meeting Tuesday morning, one official said, was, "Keep working hard. Don't leave."

For those who knew and liked Flynn, another official said, "it's sad. He's a good man, and I hate to see this."

Various accounts of the Flynn saga offered by White House officials in recent days have added to confusion about how the administration viewed Flynn's actions, who knew what and when they knew it.

News accounts about a Flynn-Kislyak conversation in late December - the day before Obama announced new sanctions related to Russian election interference - first surfaced in a David Ignatius column in The Washington Post on Jan. 12. Asked the next day whether they had talked about the sanctions in light of Trump's campaign and post-election pledges to better relations with Russia, White House officials said the subject had not been discussed.

Three days later, Pence told CBS's "Face the Nation" that Flynn had assured him personally there was no conversation about sanctions. Spicer offered similar assurances in a subsequent White House briefing.

On Jan. 24 or 25, based on discrepancies between comments by Pence and Spicer and what they knew from regular intercepts of Kislyak's calls, FBI agents interviewed Flynn. Details of that interview, first reported Tuesday by the New York Times, are unknown but they could expose Flynn to possible charges if he denied that he had discussed sanctions with Kislyak. That interview was followed by the Justice notice to McGahn, who immediately informed Trump and others, officials said.