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.

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.

In his resignation letter, Flynn said in the fast pace of events he inadvertently briefed Vice President Pence with incomplete information about his Russian contacts.

Gen. Keith Kellogg has been named his interim replacement.

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.”

The White House says President Donald Trump is "evaluating the situation" involving his national security adviser Michael Flynn.

Trump spokesman Sean Spicer says the president is speaking with Vice President Mike Pence and others about the matter.

Pence publicly vouched that Flynn did not discuss sanctions with Russia's ambassador to the U.S. before the inauguration. But Flynn has since told the White House sanctions may have come up.

Trump has said nothing publicly about Flynn since the Washington Post reported last week that Flynn did discuss sanctions with the Russian official.

White House national security adviser Michael Flynn has apologized to Vice President Mike Pence for the controversy surrounding his discussions with the Russian ambassador to the U.S., an administration official says.

Pence said in televised interviews that Flynn had not discussed sanctions during his pre-inauguration calls with the Russian envoy. Flynn has since told the White House that sanctions may have come up.

Administration officials have said Pence was relying on information from Flynn.

The official was not authorized to discuss the matter publicly and insisted on anonymity.