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 said earlier the White House had been warned about the contacts last month and had been told Mr Flynn might 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.