It was discovered that the conversation had taken place before Mr. Trump took office; and Mr. Flynn had misled Vice President Mike Pence about the conversations.
Mr. Flynn’s resignation came hours after it was reported that the Justice Department had warned the White House weeks ago that Flynn could be vulnerable to blackmail for contacts with Russian ambassador, Sergey Kislyak, before Mr. Trump took power on Jan. 20.
His departure is a sobering development in Mr. Trump’s young presidency, a 24-day period during which his White House has been repeatedly distracted by miscues and internal dramas.
The departure could also slow Mr. Trump’s bid to warm up relations with Russian President Vladimir Putin.
Mr. Flynn submitted his resignation hours after Mr. Trump, through a spokesman, pointedly declined to publicly back Mr. Flynn, saying he was reviewing the situation and talking to Mr. Pence.
Mr. Flynn promised Mr. Pence that he had not discussed U.S. sanctions with the Russians, but transcripts of communications described by U.S. officials showed that the subject had come up in conversations between him and the Russian ambassador.
Such contacts could potentially be in violation of a law banning private citizens from engaging in foreign policy known as the Logan Act.
Mr. Pence had defended Mr. Flynn in television interviews and was described by administration officials as upset about being misled.
“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.
“I have sincerely apologised to the president and the vice president, and they have accepted my apology,” Flynn said in his resignation letter.
Keith Kellogg, a retired army general, who has been chief of staff of the White House National Security Council, was named the acting national security adviser while Mr. Trump determines who should fill the position.
Mr. Kellogg, retired General David Petraeus, a former CIA director, and Robert Harward, a former deputy commander of U.S. Central Command, are under consideration for the position, a White House official said.
Mr. Harward was however described by officials as the leading candidate.
Sally Yates, the then-acting U.S. attorney general, had told the White House late last month that she believed Mr. Flynn had misled them about the nature of his communications with the Russian ambassador.
She said Mr. Flynn might have put himself in a compromising position, possibly leaving himself vulnerable to blackmail, the official said.
Ms. Yates was later fired for opposing Mr. Trump’s temporary entry ban for people from seven mostly Muslim nations.
A U.S. official, describing the intercepted communications, said Mr. Flynn did not make any promises about lifting the sanctions.
However, he indicated that sanctions imposed by President Barack Obama on Russia for its Ukraine incursion “would not necessarily carry over to an administration seeking to improve relations between the U.S. and Russia.”
Mr. Flynn, a retired U.S. Army lieutenant general, was an early supporter of Mr. Trump and shares his interest in shaking up the establishment in Washington.
He frequently raised eyebrows among Washington’s foreign policy establishment for trying to persuade Mr. Trump to warm up U.S. relations with Russia.
A U.S. official said Mr. Flynn’s departure, coupled with Russia’s aggression in Ukraine and Syria and Republican congressional opposition to removing sanctions on Russia, removes Mr. Trump’s most ardent advocate of taking a softer line towards Mr. Putin.
Mr. Flynn’s leaving “may make a significant course change less likely, at least any time soon,” the official said.
Another official said Mr. Flynn’s departure may strengthen the hands of some cabinet secretaries, including Defence Secretary, Jim Mattis and Secretary of State, Rex Tillerson.
However, the second official said Mr. Flynn’s exit could also reinforce the power of presidential aides , Steve Bannon and Stephen Miller, whom he described as already having the president’s ear.
Congressional Democrats expressed alarm at the developments surrounding Mr. Flynn and called for a classified briefing by administration officials to explain what had happened.
“We are communicating this request to the Department of Justice and FBI this evening,” said Democratic representatives John Conyers of Michigan and Elijah Cummings of Maryland.
U.S. Representative Adam Schiff of California, ranking Democrat on the House Permanent Select Committee on Intelligence, said Mr. Flynn’s departure does not end the questions over his contacts with the Russians.
“The Trump administration has yet to be forthcoming about who was aware of Flynn’s conversations with the ambassador and whether he was acting on the instructions of the president or any other officials, or with their knowledge,” Mr. Schiff said.
The committee’s chairman, Republican Devin Nunes, thanked Mr. Flynn for his service.
“Washington D.C. can be a rough town for honourable people, and Flynn, who has always been a soldier, not a politician, deserves America’s gratitude and respect,” he said.
(NAN/REUTERS)