US National Security Adviser Michael Flynn has resigned over allegations he discussed US sanctions with Russia before Donald Trump took office.
Mr Flynn is said to have misled officials about his call with Russia’s ambassador before his own appointment.
It is illegal for private citizens to conduct US diplomacy.
US reports earlier said the White House had been warned about the contacts last month and was told Mr Flynn may be vulnerable to Russian blackmail.
The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.
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In his letter of resignation (PDF), Mr Flynn said he had “inadvertently briefed the vice-president-elect and others with incomplete information regarding my phone calls with the Russian ambassador” late last year.
A White House statement said Lt Gen Joseph Keith Kellogg had been appointed as interim replacement for the post.
Mr Flynn, a retired Army lieutenant general, initially denied having discussed sanctions with Ambassador Sergei Kislyak, and Vice-President Mike Pence publicly denied the allegations on his behalf.
But he came under further pressure on Monday when details of his phone call emerged in US media, as well as reports the justice department had warned the White House about him misleading senior officials and being vulnerable to Russian blackmail.
According to the Washington Post, the message was delivered by then-acting attorney general Sally Yates, who was subsequently dismissed by President Trump for opposing his controversial travel ban.
What are the Russians saying?
Kremlin spokesman Dmitry Peskov said Russia would not be commenting on the resignation.
“This is the internal affair of the Americans, the internal affair of the Trump administration. It’s nothing to do with us,” he added.
Other Russian lawmakers have spoken out in defence of Mr Flynn, with Senator Alexei Pushkov tweeting that he had been “forced to resign not because of his mistake but because of a full-fledged aggressive campaign”.
“Trump is the next target,” he tweeted.
Is Trump implicated? – Anthony Zurcher, BBC North America reporter
From inauguration to full-blown scandal and a high-level resignation in 24 days. That simply has to be some kind of record.
Donald Trump never does anything small. If his administration is going to have a political crisis, why waste any time?
From the day he was announced as Mr Trump’s national security adviser, there were concerns about Michael Flynn’s questionable contacts with Russia both before and after November’s election.
The ground crumbled beneath his feet only recently, however, after revelations that his conversation with a Russian ambassador included talk of US-imposed sanctions. The mortal blow came late on Monday, with reports that Obama-era government officials had warned the Trump White House about the details of these calls in January.
Now Mr Flynn has been cut loose, but that may not be enough to staunch the bleeding.
Congressional Democrats – and perhaps some Republicans – will want to find out who was informed about Mr Flynn’s contradictory stories and why nothing was done earlier. How far up the chain of command does it go?
All of this has some observers dusting off language from the mother of all presidential scandals, Watergate.
What did the president know, and when did he know it?
Democrats demand investigation
Senior Democrat Adam Schiff said Mr Flynn’s departure would not end questions about any contacts between Donald Trump’s campaign and Russia.
Congressional democrats John Conyers and Elijah Cummings have demanded a classified briefing to Congress on Michael Flynn by the Justice department and FBI.
“We in Congress need to know who authorised his actions, permitted them, and continued to let him have access to our most sensitive national security information despite knowing these risks,” their statement said.
Several House Democrats had already called on Oversight Committee Chairman Jason Chaffetz to launch an investigation into Mr Flynn’s ties to Russia.
Democrat Hillary Clinton, who ran against Donald Trump for presidency, retweeteda former aide who wrote “Dear Mike Flynn & Mike Flynn Jr. What goes around COMETS (sic) around” – in reference to the sacking of Mr Flynn’s son over a fake news story dubbed ‘Pizzagate’.
Mr Flynn had joined in with the “lock her up” chants against Mrs Clinton during the presidential campaign over her use of email when she was secretary of state.
Flynn’s son sacked over ‘Pizzagate’ tweets
Who is Michael Flynn?
Mr Flynn, who was previously fired by Barack Obama as head of the Defense Intelligence Agency, was an ardent supporter of Mr Trump during the campaign.
He became a close ally of both the president and his chief strategist, Steve Bannon.
He encouraged tougher policies on Iran and a softer policy on Russia, but questions were raised about his perceived closeness to Moscow.
More on Michael Flynn
How Flynn floundered
- Mr Flynn is known to have spoken with Russian ambassador Sergei Kislyak several times by phone in December;
- Mr Flynn denied that he and Mr Kislyak had discussed US sanctions and Vice-President Mike Pence also denied the claims on his behalf;
- A spokesman for Mr Flynn then backtracked, telling reporters the adviser “couldn’t be certain” he had not discussed the sanctions;
- On Monday, White House counsellor Kellyanne Conway said Mr Trump had “full confidence” in Mr Flynn;
- White House spokesman Sean Spicer later said the president was “evaluating the situation”. Hours later, Mr Flynn resigned
Who’s the man who replaces him?
Retired Lt Gen Joseph Keith Kellogg has been appointed acting national security adviser, and is far from a newcomer to the Trump team.
He brings more than 30 years’ experience in the army, and served in Vietnam, Cambodia, Panama and the Gulf.
During the Iraq war, he helped manage the coalition authority running the country in 2003 and 2004, before working for a defence contractor, according to Bloomberg.
More recently, he advised Mr Trump on national security issues during his campaign, and went on to be appointed chief of staff in the new administration’s National Security Council.
Former CIA director retired General David Petraeus and Robert Harward, a former deputy commander of US Central Command are also under consideration for the post, a White House official has said.