These calls were to facilitate a smooth transition and begin to build the necessary relationships between the President, his advisors and foreign leaders. In his resignation letter, Flynn noted that he apologized to Trump and Pence and that they accepted the apology. The same day, Flynn speaks to Russian ambassador Sergey Kislyak by phone.
Keith Kellogg, the chief of staff, will step in as the acting national security adviser and head of the National Security Council.
Just six days into his presidency, Donald Trump was informed his national security adviser had misled his vice president about contacts with Russian Federation.
Lindsey Graham said Congress needed to know whether he had been acting with direction from the president or others.
He said the general was loyal to him during the campaign, and he appreciated his strength and intelligence.
The US Senate's top Republican Mitch McConnell said it was "highly likely" that Flynn would have to testify before an intelligence panel, potentially heaping pressure on Trump.
The unprecedented scrutiny of ties between the Trump administration and Moscow is in and of itself politically damaging for the White House.
In his resignation letter, Mr Flynn said he held numerous calls with the Russian ambassador to the USA during the transition and gave "incomplete information" about those discussions to Vice-President Mike Pence.
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Flynn first meets Trump, according to an interview he gave to the Washington Post.
Trump initially thought Flynn could survive the controversy, according to a person with direct knowledge of the president's views, but a pair of explosive stories in The Washington Post in recent days made the situation untenable. Though the White House is insisting we all "move on" now that Flynn's resigned, there are still some very troubling questions we should demand are answered.
But on Friday the Washington Post reported that it had sources vouching Flynn had discussed sanctions with the Russian ambassador to the United States while Barack Obama was still president. "He's getting input, he's looking at the situation". "General Flynn does enjoy the full confidence of the president", Conway told MSNBC.
Flynn is said to have given " incomplete information" ro the white house officials about his discussions with the Russian ambassador over USA sanctions against Moscow.
The Flynn incident also highlights some broader political problems for Trump.
A White House spokeswoman declined to respond to questions from POLITICO about the search for potential Flynn successors.
"We're not easing", Trump spokesman Sean Spicer said.
Petraeus did not respond to an email seeking comment. But it also might rob Trump of his ability to lift sanctions on Russian Federation, one of his most widely anticipated maneuvers created to reset Washington's relationship with the Kremlin.
Both Democratic and Republican lawmakers had urged Flynn to resign.
Bennet, a Democrat, has joined with fellow party members in calling for a new investigation into Flynn's conversation - and who in the nascent administration knew about it. "All quiet in my nets", the retired admiral said.
Several groups are investigating whether wrongdoing took place including The House Permanent Select Committee on Intelligence.
Mr. Flynn's resignation comes as the President and his top advisers seek to steady the White House after a rocky start.
Trump asked Flynn to resign on Monday after the retired army general admitted having misled Pence on whether his discussions in December with Russian Ambassador Sergey Kislyak involved United States sanctions on Moscow.