United States President Donald Trump knew for weeks that national security adviser Michael Flynn had misled the White House about his contacts with Russia but did not immediately force him out, an administration spokesman has said.

Key points: Democrats, Republicans say Americans deserve to find out what Trump knew

Democrats, Republicans say Americans deserve to find out what Trump knew Mr Flynn's discussions may violate the Logan Act, banning private citizens from diplomacy

Mr Flynn's discussions may violate the Logan Act, banning private citizens from diplomacy Officials suggest Mr Flynn advised Russia not to retaliate to Obama's sanctions to improve relation with Trump's administration

Officials suggest Mr Flynn advised Russia not to retaliate to Obama's sanctions to improve relation with Trump's administration Vice Admiral Robert Harward is the leading candidate to replace Flynn

Mr Trump was informed in late January that Mr Flynn had not told Vice-President Mike Pence the whole truth about conversations he had with Russia's ambassador to the United States before Mr Trump took office, White House spokesman Sean Spicer said.

Mr Flynn quit on Tuesday after Mr Trump asked for his resignation, Mr Spicer said.

"The issue pure and simple came down to a matter of trust," Mr Spicer said.

The departure was another disruption for an administration already repeatedly distracted by miscues and internal dramas since the Republican businessman assumed the presidency on January 20.

US lawmakers, including some leading Republicans, called for a deeper inquiry into not just Mr Flynn's actions but broader White House ties to Russia.

Mr Trump has long said that he would like improved relations with Russian President Vladimir Putin.

Representative Adam Schiff, the top Democrat on the House of Representatives Intelligence Committee, said Mr Trump only moved against Mr Flynn because of media attention to the issue, and not because of concern at any wrongdoing by the former Lieutenant General.

"The reason they lost faith or trust in General Flynn only last night when they knew for weeks that he had been lying was that it became public," Mr Schiff told MSNBC.

A timeline of events outlined by Mr Spicer and a US official showed that Mr Trump had known for weeks about Mr Flynn misleading the Vice-President.

Trump held back before saying 'you're fired' to Flynn

Mr Trump, a former reality TV star whose catchphrase was "You're fired!", has often boasted of his eagerness to get rid of subordinates.

But he was not quick to fire Mr Flynn, a strong advocate of a better relations with Russia and a hard line against Islamist militants.

The Justice Department warned the White House in late January that Mr Flynn had misled Mr Pence by denying to him that he had discussed US sanctions on Russia with Ambassador Sergei Kislyak, a potentially illegal act, a US official said.

Mr Flynn did talk about sanctions with the diplomat, whose calls were recorded by US intelligence officials, the official said.

But Mr Pence went on television in mid-January and denied that Mr Flynn had discussed sanctions.

Mr Spicer stressed that the administration believed there was no legal problem with Mr Flynn's conversations with Mr Kislyak, but rather an issue over the President's trust in his adviser.

He said the Justice Department sought to notify the White House counsel on January 26 about the discrepancies in Mr Flynn's accounts.

"The White House counsel informed the President immediately," Mr Spicer said.

"The President asked them to commit a review of whether there was a legal situation there. That was immediately determined there wasn't.

"That was what the President believed at the time from what he had been told and he was proved to be correct.

"We got to a point not based on a legal issue, but based on a trust issue."

Legal fallout for Flynn?

Mr Flynn's conversations with the ambassador reportedly occurred when the Obama administration slapped sanctions on Russia. ( Reuters: Carlos Barria )

Mr Flynn's conversations with the ambassador took place around the time that then-president Barack Obama imposed sanctions on Russia, charging that Moscow had used cyber attacks to try to influence the 2016 presidential election in Mr Trump's favour.

A US official familiar with the transcripts of the calls with the ambassador said Mr Flynn indicated that if Russia did not retaliate in kind for Obama's December 29 order expelling 35 Russian suspected spies and sanctioning Russian spy agencies, that could smooth the way toward a broader discussion of improving US-Russian relations once Mr Trump took power.

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Mr Flynn's discussions with the Russian diplomat could potentially have been in violation of a law known as the Logan Act, banning private citizens from negotiating with foreign governments about disputes or controversies with the US.

However, nobody has been prosecuted in modern times under the law, which dates from 1799.

Although Mr Flynn is almost certain not to be prosecuted under the Logan Act, he could still face legal trouble if it emerges that he violated other federal laws in his communications with the Russians, said Andrew Kent, a professor at Fordham University School of Law in New York.

The Espionage Act, for example, criminalises sharing information with foreign governments.

Democrats, who do not have control of Congress, clamoured for probes into Mr Flynn, and asked how much Mr Trump knew about his connections to Russia.

Sorry, this video has expired Law professor Khaled Beydoun discusses the implications of Mr Flynn's resignation.

US Senate Democratic leader Chuck Schumer called for an investigation of potential criminal violations surrounding the resignation of Mr Flynn and said senior Mr Trump administration officials should face tough questions.

"What I am calling for is an independent investigation with executive authority to pursue potential criminal actions," Mr Schumer said, adding that such a probe could not be led by US Attorney-General Jeff Sessions or White House lawyers.

Two leading Republicans in the Senate, Bob Corker and John Cornyn, said the intelligence committee should investigate Mr Flynn's contacts with Russia.

But the highest-ranking Republican in Congress, House of Representatives Speaker Paul Ryan, sidestepped questions about whether lawmakers should look into Mr Flynn's Russia ties, adding that he would leave it to the Trump administration to explain the circumstances behind Mr Flynn's departure.

A broader investigation of the White House and its ties to Russia is not possible without the cooperation either of the Justice Department or the Republican-led Congress.

"Nothing is going to happen without some Republicans moving," Professor Kent said.

Russia's aggression in Ukraine and Syria and Republican congressional opposition to removing sanctions on Russia make any White House attempt to embrace Mr Putin problematic.

Senator John McCain, a leading Republican voice on foreign relations, said Mr Flynn's resignation raised questions about the administration's intentions toward Mr Putin's Russia.

Reuters