"In the course of my duties as the incoming national security adviser, I held numerous phone calls with foreign counterparts, ministers and ambassadors," the letter said. "These calls were to facilitate a smooth transition and begin to build the necessary relationships between the President, his advisors and foreign leaders. Such calls are standard practice in any transition of this magnitude. "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. "Throughout my over 30 years of honourable military service and my tenure as a national security adviser, I've always performed my duties with the utmost of integrity and honesty to those I have served. "I am tendering my resignation, honoured to have served our nation and the American people in such a distinguished way. I am also extremely honoured to have served President Trump, who in just three weeks has reoriented American foreign policy in fundamental ways to restore America's leadership position in the world.

"As I step away once again from serving my nation in this current capacity, I wish to thank President Trump for his personal loyalty." The letter concluded with Mr Flynn saying Donald Trump and Mike Pence would go down in history as "one of the greatest presidencies in US history" as they work to "make America great again". Flynn's departure less than one month into the Trump administration marks an extraordinarily early shake-up in the president's senior team of advisers. Flynn was a loyal Trump supporter throughout the campaign, but his ties to Russia caused concern among other senior aides. Flynn initially told Trump advisers that he did not discuss sanctions with the Russian envoy during the transition. Pence, apparently relying on information from Flynn, publicly vouched for Flynn. Flynn's resignation came after it The Washington Post reported that the Justice Department warned the White House weeks ago that Flynn could be vulnerable to blackmail for contacts with Russian officials before Trump took power on January 20.

Sally Yates, the then-acting US attorney general, told the White House late last month that she believed Flynn had misled them about the nature of his communications with the Russian ambassador to the United States. She said Flynn might have put himself into a compromising position. Yates was later fired for opposing Trump's temporary travel ban for people from seven mostly Muslim nations. Flynn later told White House officials that he may have discussed sanctions with the ambassador. On Monday, a top White House adviser had said that Flynn enjoyed the "full confidence" of President Donald Trump. The resignation comes amid reports that US security agencies had taken to withholding sensitive intelligence from their presidential briefings, because this White House is not to be trusted with the nation's secrets.

A United States Army lieutenant general Flynn was head of the Defence Intelligence Agency until he was fired by President Barack Obama for a number of reasons, including mismanagement. Retired General Keith Kellogg, who has been chief of staff of the White House National Security Council, was named the acting national security adviser while Trump determines who should fill the position. Retired General David Petraeus, a former CIA director, is under consideration for the position, a White House official said. Loading More to come

Reuters, Washington Post, AAP