US PRESIDENT Donald Trump asked for National Security Adviser Michael Flynn's resignation, with his spokesman saying it "was not a legal issue but a trust issue."

White House press secretary Sean Spicer told reporters that Mr Trump had been concerned that General Flynn had misled Vice President Mike Pence over his contacts with Russian officials before Mr Trump took office.

Mr Trump had been reviewing Gen. Flynn's situation for a few weeks, he said. Amid congressional calls for an investigation into the extent to which Gen. Flynn discussed the possibility of lifting US sanctions on Russia with the Russian ambassador to Washington, Mr Spicer said the White House legal counsel had reviewed the situation and believed Gen. Flynn's case was viewed "not as a legal issue but a trust issue".

Mr Spicer said Mr Trump was informed of Gen. Flynn's conversations with Russian ambassador Sergei Kislyak and asked counsel to review the case.

"The erosion of that trust, frankly, was the issue," Mr Spicer said.

He said Mr Trump had not told Gen. Flynn to discuss US sanctions on Russia with the Russian diplomat.

Mr Trump blasted the leaks coming out of his White House.

"The real story here is why are there so many illegal leaks coming out of Washington? Will these leaks be happening as I deal on N. Korea etc?" President Trump tweeted on Monday.

North Korea tested a ballistic missile over the weekend while Mr Trump was at his Florida resort hosting Japanese Prime Minister Shinzo Abe.