President Donald Trump's embattled national security adviser Michael Flynn has resigned, following reports that he had misled Vice President Mike Pence and other officials about his contacts with Russia.
His departure comes after less than a month in office.
In a resignation letter, Mr Flynn said he held numerous calls with the Russian ambassador to the US during the transition and gave "incomplete information" about those discussions to Mr Pence.
The vice president, apparently relying on information from Mr Flynn, initially said the national security adviser had not discussed sanctions with the Russian envoy, though Mr Flynn later conceded the issue may have come up.
Mr Trump named retired Lieutenant General Keith Kellogg as the acting national security adviser.
Lt Gen Kellogg had previously been appointed the National Security Council chief of staff and advised Mr Trump on national security issues during the campaign.
The US Justice Department warned the Trump administration weeks ago that contradictions between the public depictions and the actual details of the calls could leave Mr Flynn in a compromised position, sources told The Associated Press.
The White House has been aware of the warnings for "weeks," an administration official said, though it was unclear whether Mr Trump and Mr Pence had been alerted.
Mr Flynn apologised to Mr Pence last week, following a Washington Post report asserting that the national security adviser had discussed sanctions with the Russian envoy.
White House spokesman Sean Spicer said Mr Trump was consulting with Mr Pence on Monday about his conversations with the national security adviser.
Asked whether the president had been aware that Mr Flynn might discuss sanctions with the Russian envoy, Mr Spicer said: "No, absolutely not."
Mr Flynn's discussions with the Russian raised questions about whether he had offered assurances about the incoming administration's new approach.
Such conversations would breach diplomatic protocol and possibly violate the Logan Act, a law aimed at keeping citizens from conducting diplomacy.
Mr Flynn was a loyal Trump supporter during the campaign, but he is viewed sceptically by some in the administration's national security circles, in part because of his ties to Russia.
In 2015, Mr Flynn was paid to attend a gala dinner for Russia Today, a Kremlin-backed television station, and sat next to Russian President Vladimir Putin during the event.
The controversy surrounding Mr Flynn comes as the administration grapples with a series of national security challenges, including North Korea's reported ballistic missile launch.
The White House is also dealing with fallout from the troubled roll-out of Mr Trump's immigration executive order, which has been blocked by the courts.
The order was intended to suspend the nation's refugee programme and bar citizens of seven Muslim-majority countries from entering the United States.
A judge on Monday granted a preliminary injunction barring the Trump administration from implementing its travel ban in Virginia, adding another ruling to those already in place.
The ruling is significant as US District Judge Leonie Brinkema found that an unconstitutional religious bias is at the heart of the ban, and it therefore violates First Amendment prohibitions on favouring one religion over another.
A California appeals court has already upheld a national temporary restraining order stopping the government from implementing the ban.