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.
The White House has appointed Lt Gen Joseph Keith Kellogg 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.
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