Kellyanne Conway, a close aide to Trump, had said Monday that Flynn continued to have the "full confidence" of the president. On Tuesday, she said in televised interviews that Trump had supported Flynn out of loyalty but that the situation reached a "fever pitch" and had become "unsustainable."
"By night's end, Mike Flynn had decided it was best to resign. He knew he'd become a lightning rod, and he made that decision," Conway told NBC's "Today" show.
When asked why the White House didn't move sooner after being warned by the Justice Department that Flynn was at risk of blackmail, Conway was vague: "As time wore on, obviously the situation became unsustainable," she repeated.
She added: "We're moving on."
Trump named retired Lt. Gen. Keith Kellogg as the acting national security adviser. Kellogg had previously been appointed the National Security Council chief of staff and advised Trump during the campaign. Trump is also considering former CIA Director David Petraeus and Vice Admiral Robert Harward, a U.S. Navy SEAL, for the post, according to a senior administration official.
A U.S. official told The Associated Press that Flynn was in frequent contact with Ambassador Sergey Kislyak on the day the Obama administration slapped sanctions on Russia for election-related hacking, as well as at other times during the transition.
An administration official and two people with knowledge of the situation confirmed the Justice Department warnings on condition of anonymity because they were not authorized to discuss the matter publicly. It was unclear when Trump and Pence learned about the Justice Department outreach.
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 Flynn had indeed spoken about sanctions with the Russian ambassador.
Trump never voiced public support for Flynn after that initial report but continued to keep his national security adviser close.
The White House officials sent contradictory messages, meantime, about Flynn's job status. While Conway was remarking that Trump had "full confidence" in the retired general, White House press secretary Sean Spicer said the president was "evaluating the situation" and consulting with Pence about his conversations with the national security adviser.
Asked whether the president had been aware that Flynn might have planned to discuss sanctions with the Russian envoy, Spicer said, "No, absolutely not."
The Kremlin had confirmed that Flynn was in contact with Kislyak but denied that they talked about lifting sanctions. On Tuesday, Russian lawmakers mounted a fierce defense of Flynn.
Konstantin Kosachev, chairman of the foreign affairs committee at the upper chamber of the Russian parliament, said in a post on Facebook that firing a national security adviser for his contacts with Russia is "not just paranoia but something even worse." Kosachev also expressed frustration at the Trump administration:
"Either Trump hasn't found the necessary independence and he's been driven into a corner... or russophobia has permeated the new administration from top to bottom," he said.
Kosachev's counterpart at the lower chamber of the Russian parliament, Alexei Pushkov, tweeted shortly after the announcement that "it was not Flynn who was targeted but relations with Russia."
California Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, said Flynn's resignation "does not end questions over his contacts with the Russians." He said the White House has yet to be forthcoming about whether Flynn was acting at the behest of the president or others.
Associated Press writers Catherine Lucey and Matthew Daly in Washington and Jonathan Lemire in New York contributed to this report.
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