Donald Trump’s national security adviser, Michael Flynn, has resigned over his contacts with Russia, the White House has announced.
Mr Flynn is alleged to have discussed US sanctions with the Russian ambassador before Mr Trump took office.
He is said to have misled officials about the conversation.
Earlier, US media reported that the Justice Department had warned the White House about the contacts late last month.
They said that Mr Flynn might be vulnerable to Russian blackmail.
Senior Democrats had called for Mr Flynn to be fired.
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.
In his letter of resignation, Mr Flynn said he had “inadvertently briefed the vice-president-elect and others with incomplete information regarding my phone calls with the Russian ambassador”.
A White House statement said Lt Gen Joseph Keith Kellogg had been appointed as interim replacement.
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.
However, Mr Flynn later told the White House that sanctions may have been discussed.
From inauguration to full-blown scandal punctuated by 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?
Kremlin spokesman Dmitry Peskov told reporters on Monday that Mr Flynn and Mr Kislyak did not discuss lifting sanctions.
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.
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.
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.