United States national security adviser Michael Flynn has resigned over his contact with Russia, less than a month into the Trump administration.
The White House confirmed the resignation, saying General Keith Kellogg had been appointed acting national security adviser in the meantime.
Mr Flynn has faced a growing firestorm over the past day, over reports he discussed sanctions with Russian ambassador Sergei Kislyak in the weeks before Mr Trump's inauguration.
He denied talking about this issue but later said he could not be certain.
Any discussion of sanctions could have violated laws against private citizens engaging in foreign policy.
Kremlin spokesman Dmitry Peskov told reporters on Monday that Mr Flynn and Mr Kislyak did not discuss lifting sanctions.
In a resignation letter, contained within the White House statement announcing Mr Flynn's resignation, Mr Flynn said he did not fully brief other members of the administration.
"Unfortunately ... I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador.
"I have sincerely apologised to the President and the Vice President, and they have accepted my apology," Mr Flynn wrote.
He was "extremely honoured" to have served Mr Trump.
"This team will go down in history as one of the greatest presidencies in US history," he wrote.