National security adviser Michael Flynn has resigned after reports he misled Trump administration officials about his contacts with Russia's ambassador to the U.S.
Retired Lt. Gen. Josheph Keith Kellogg, Jr. was named acting National Security Advisor late Monday.
Flynn's departure less than one month into the Trump administration marks an extraordinarily early shakeup in the president's senior team of advisers. Flynn was a loyal Trump supporter throughout the campaign, but his ties to Russia caused concern among other senior aides.
Flynn initially told Trump advisers that he did not discuss sanctions with the Russian envoy during the transition. Vice-President Mike Pence, apparently relying on information from Flynn, publicly vouched for the national security adviser.
Flynn later told White House officials that he may have discussed sanctions with the ambassador.
Two people familiar with the situation say the Justice Department warned the Trump administration about Michael Flynn's contacts with Russia.
One of the people says the Justice Department told the administration there was a discrepancy between what the White House was saying publicly about Flynn's contacts and the facts of what occurred.
The person says the Justice Department was concerned that Flynn could be in a compromised position.
A Trump administration official says the White House has been aware of the Justice Department warning for "weeks." That official would not say if the president had been briefed on the matter.
The two people were not authorized to discuss the matter by name and spoke on condition of anonymity.
The Washington Post first reported the communications with the Justice Department.
A White House spokesman says no classified material was discussed publicly at the Mar-a-Lago resort over the weekend.
Sean Spicer says President Donald Trump was briefed in a classified room after North Korea's latest missile test Saturday. Trump drew scrutiny after social media posts appeared to show Trump conducting national security business in an area accessible to the public.
Spicer says Trump was briefed before and after dinner Saturday. But he says that during dinner Trump and U.S. and Japanese officials were discussing plans for a press conference later that night.
Spicer says the activity at dinner was "literally a discussion of logistics."