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.
The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.
What did Mr Flynn say about the phone call?
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 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.
However, Mr Flynn later told the White House that sanctions may have been discussed.
Kremlin spokesman Dmitry Peskov told reporters on Monday that Mr Flynn and Mr Kislyak did not discuss lifting sanctions.
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.