White House 'was warned over Michael Flynn's Russia contact'
The White House was warned over links between its national security adviser and Russia, US media say.
Michael Flynn is under scrutiny amid reports he spoke to the Russian ambassador to the US about sanctions before Donald Trump's inauguration.
Associated Press and the Washington Post say the Justice Department alerted the White House over its concerns.
They say the department warned of inconsistencies in what Mr Flynn said about his contact with the ambassador.
It is illegal for private citizens to conduct US diplomacy, and the calls happened late last year, before Mr Flynn was appointed to the administration.
AP said the White House was warned over inconsistencies in his story "weeks ago". The government and Mr Flynn have not responded to the latest allegations.
Mr Flynn, a retired Army lieutenant general, initially denied having discussed sanctions with the ambassador, and Vice-President Mike Pence publicly denied the allegations on his behalf.
However, Mr Flynn later told the White House they may have been discussed.
The Washington Post said former Acting Attorney General Sally Yates told the White House that their national security adviser "had put himself in a compromising position" by speaking to the Russians before he was allowed, and that the Justice Department had learned he had misled Mr Pence.
Last month, Ms Yates was fired by Mr Trump after refusing to enforce his ban on people from seven Muslim-majority countries.
White House spokesman Sean Spicer said on Monday afternoon: "The president is evaluating the situation.
"He is speaking to the vice-president relative to the conversation the VP had with General Flynn and also speaking to various other people about what he considers the single most important subject there is - our national security."
Mr Spicer's statement came soon after White House counsellor Kellyanne Conway said Mr Flynn had "the full confidence of the president".
"It's a big week for General Flynn. He's the point of contact for many of these foreign visits," she told MSNBC.
Mr Flynn has apologised to the vice-president over the controversy surrounding his conversations with Russian ambassador Sergei Kislyak.
Kremlin spokesman Dmitry Peskov told reporters on Monday that Mr Flynn and Mr Kislyak did not discuss lifting sanctions.
According to Politico, Mr Trump's adviser and son-in-law, Jared Kushner, is scouting for possible candidates to replace Mr Flynn.
How the controversy unfolded
Mr Flynn is known to have spoken with Mr Kislyak several times by phone in December.
With the support of Vice-President Pence, Mr Flynn denied that he and Mr Kislyak had discussed US sanctions imposed over Russia's actions in Ukraine and alleged hacking of the US Democratic Party.
But nine current and former officials later told the Washington Post the issue had been discussed.
A spokesman for Mr Flynn subsequently backtracked, telling reporters the adviser now said he "couldn't be certain" he had not discussed the sanctions, prompting speculation that he may have misled the vice-president.
Mr Pence and Mr Flynn reportedly spoke twice on Friday when he privately apologised to the vice-president, according to an administration official.
What is the president's view?
Mr Trump has yet to publicly comment on the controversy, but was with Mr Flynn over the weekend at his club in Florida, Mar-a-Lago.
Mr Flynn appeared in the front row at President Trump's news conference on Monday with Canadian Prime Minister Justin Trudeau.
Mr Trump was not asked about Mr Flynn by the two reporters he called on, and evaded follow-up questions about the controversy as he left the room.
Asked if the president was aware that Mr Flynn could discuss sanctions with the Russian ambassador, Mr Trump's spokesman, Sean Spicer, said: "No, absolutely not. No way."
What's the reaction on Capitol Hill?
Democratic House Minority Leader Nancy Pelosi on Monday called for Mr Trump to fire Mr Flynn, tweeting that he "cannot be trusted to serve America's best interests and national security instead of Russia's".
Several House Democrats have called on Oversight Committee Chairman Jason Chaffetz to launch an investigation into Mr Flynn's ties to Russia.
Republican Senator Susan Collins of Maine said it would also be "troubling'' if Flynn had been negotiating with a foreign government before taking office.
Who is Michael Flynn?
He was an ardent supporter of Mr Trump during the campaign, and he has become a close ally of both the president and the president's chief strategist, Steve Bannon.
He has encouraged tougher policies on Iran and a softer policy on Russia, but questions have been raised about his perceived closeness to Moscow.
He attended a banquet last year held in honour of the Russian government, where he sat two seats away from Vladimir Putin.
A top aide on Mr Flynn's team, Robin Townley, was this week denied security clearance by the CIA, preventing Mr Townley from taking up a post in Africa for which Mr Flynn had recommended him.
Mr Flynn's son has also attracted unwelcome attention - and reportedly a personal rebuke from Mr Trump after tweeting about the so-called Pizzagate fake news story.