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.
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.
A number of senior Democrats had called for Mr Flynn to be fired.
The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.
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”.