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.
What did Mr Flynn say about the phone call?
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.
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.
But he came under further pressure on Monday when details of his phone call emerged in US media, as well as reports the justice department had warned the White House about him misleading senior officials and being vulnerable to Russian blackmail.
According to the Washington Post, the message was delivered by then-acting attorney general Sally Yates, who was subsequently dismissed by President Trump for opposing his controversial travel ban.
What are the Russians saying?
Kremlin spokesman Dmitry Peskov said Russia would not be commenting on the resignation.
"This is the internal affair of the Americans, the internal affair of the Trump administration. It's nothing to do with us," he added.
Other Russian lawmakers have spoken out in defence of Mr Flynn, with Senator Alexei Pushkov tweeting that he had been "forced to resign not because of his mistake but because of a full-fledged aggressive campaign".
"Trump is the next target," he tweeted.
(Published by BBC - February 14, 2017)