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.
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.
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.
Leading members of the US Republican Party have joined calls for a wide investigation into the former national security adviser's links with Russia.
On Tuesday, a White House spokesman said Mr Trump knew weeks ago there were problems with the Russia phone calls.
The development came as the New York Times reported that phone records and intercepted calls show members of Mr Trump's presidential campaign, as well as other Trump associates, "had repeated contacts with senior Russian intelligence officials in the year before the election".
However, officials spoken to by the newspaper said they had not yet seen evidence that the Trump campaign colluded with Russia on the hacking of the Democratic National Committee or to influence the election.
As well as an FBI investigation, both the Senate and House intelligence committees are already examining Russian involvement in the election. It is not yet clear whether the latest claims will be included in their scope.
Even if the Senate keeps approving his appointments?
It's about time they started tearing each other to pieces.
President Donald Trump has lashed out at the US intelligence community and media after new reports of contacts between members of his team and Russia.
Mr Trump accused the National Security Agency (NSA) and FBI of giving out information illegally.
It's not just the party politicians either, he now seems to be looking for a fight with the intelligence services.
Blood is in the water now. The sharks will come.
What's been rumoured is likely very tame compared to the real dirt.