It has been confirmed President Donald Trump asked his national security advisor to resign.
President Trump ousted his national security adviser because he lost trust in him, not for any legal concern, Mr Trump's spokesman said.
White House press secretary Sean Spicer said that the "erosion of that trust" over the circumstances surrounding retired general Michael Flynn's calls with the Russian ambassador to the US created "a critical mass and an unsustainable situation".
Mr Flynn's ouster appeared to be driven more by the idea that he had misled Vice President Mike Pence and other officials than by the content of his discussions with Russia's ambassador.
Still, the matter deepened questions about Mr Trump's friendly posture towards Russia.
"This was an act of trust - whether or not he misled the vice president was the issue and that was ultimately what led to the president asking for and accepting the resignation of General Flynn," Mr Spicer said.
Mr Flynn's resignation came after reports that the Justice Department had alerted the White House weeks ago that there were contradictions between Trump officials' public accounting of the Russia contacts and what intelligence officials knew to be true based on routine recordings of communications with foreign officials who are in the US.
Mr Spicer said the White House counsel's office reviewed the situation after it was flagged by the Department of Justice, and along with the president, the counsel determined that it did not pose a legal problem.
He declined to comment on whether anyone at the White House had read transcripts of the calls between Mr Flynn and the ambassador.
The revelations were another destabilising blow to an administration that has already suffered a major legal defeat on immigration, botched the implementation of a signature policy and stumbled through a string of embarrassing public relations missteps.
White House officials have not said when Mr Trump was told of the Justice Department warning or why Mr Flynn had been allowed to stay on the job with access to a full range of intelligence materials.
Senator Lindsey Graham, a long-time Russia critic, said Congress needs to know what Mr Flynn discussed with the ambassador and why.
"The idea that he did this on his own without any direction is a good question to ask," Mr Graham added.
Mr Pence and others, apparently relying on information from Mr Flynn, had said the national security adviser did not discuss US economic sanctions against Russia with the Russian envoy during the American presidential transition.
Mr Flynn later told officials the sanctions may have been discussed, the latest change in his account of his pre-inauguration discussions with Ambassador Sergey Kislyak.
Such conversations would breach diplomatic protocol and possibly violate the Logan Act, a law aimed at keeping private citizens from conducting US diplomacy.
The Justice Department had warned the White House late last month that Mr Flynn could be at risk for blackmail because of contradictions between his public depictions of the calls and what intelligence officials knew.
Asked whether the president had been aware that Mr Flynn might have planned to discuss sanctions with the Russian envoy, Mr Spicer said: "No, absolutely not."
House Speaker Paul Ryan said Mr Trump made the right decision in asking Mr Flynn to step down.
"You cannot have the national security adviser misleading the vice president and others," Mr Ryan said.
Mr Trump, who has been conspicuously quiet about Mr Flynn's standing for several days, wrote on Twitter on Tuesday morning that the "real story here is why are there so many illegal leaks coming out of Washington".
The real story here is why are there so many illegal leaks coming out of Washington? Will these leaks be happening as I deal on N.Korea etc?— Donald J. Trump (@realDonaldTrump) February 14, 2017
He ignored questions about Mr Flynn from reporters during an education event at the White House on Tuesday morning.
Mr Trump named retired Lieutenant General Keith Kellogg as the acting national security adviser.
Mr Kellogg had previously been appointed the National Security Council chief of staff and advised Mr Trump during the campaign.
Mr Trump is also considering former CIA director David Petraeus and Vice Admiral Robert Harward, a US Navy SEAL, for the post, according to a senior administration official.
Mr Kellogg convened a brief meeting of the National Security Council staff on Tuesday morning and urged them to continue with business as usual.
Staffers have been told that Mr Flynn's deputy, KT McFarland, a former Fox News analyst, is expected to stay at the White House.
A US official told The Associated Press that Mr Flynn was in frequent contact with Mr Kislyak on the day the Obama administration slapped sanctions on Russia for election-related hacking, as well as at other times during the transition.
The Washington Post was the first to report the communication between former acting attorney general Sally Yates, a holdover from the Obama administration, and the Trump White House.
The Post also first reported last week that Mr Flynn had indeed spoken about sanctions with the Russian ambassador.
Mr Trump never voiced public support for Mr Flynn after that initial report but continued to keep his national security adviser close.
Mr Flynn was part of Mr Trump's daily briefing on Monday and sat in on his calls with foreign leaders, as well as his discussions with visiting Canadian prime minister Justin Trudeau.
The Kremlin had confirmed that Mr Flynn was in contact with Mr Kislyak but denied that they talked about lifting sanctions.
On Tuesday, Russian lawmakers mounted a fierce defence of Mr Flynn.