President Trump's trust in his national security adviser eroded to the point where he asked Michael Flynn to resign, the White House says.

The US leader had been "very concerned" that the retired army lieutenant general misled Vice President Mike Pence and others, said Mr Trump's spokesman Sean Spicer.

Mr Flynn quit less than a month into the job over allegations surrounding his contact with Russia.

He apologised for giving Mr Pence and others "incomplete information" about his phone calls with Russia's ambassador Sergey Kislyak.

Mr Flynn allegedly discussed sanctions with Mr Kislyak late last year before Mr Trump was in power - and before he had officially taken up his own role.

The resignation came only hours after the Washington Post reported that that the US Justice Department had warned the White House that Mr Flynn could be vulnerable to Russian blackmail over 'mischaracterising' the details of his communications.

Under the Logan Act in American law, it is illegal for an unauthorised citizen to conduct diplomacy on behalf of the US.

But Mr Spicer said Mr Flynn had quit over a "matter of trust" rather than any legal aspects.

However, Mr Flynn may need to testify at a congressional hearing investigating Russia's alleged interference in last year's presidential election, according to Republican Senator Bob Corker.

Mr Spicer said Mr Trump was "very, very concerned" about leaks in the Flynn case.

Earlier in the day, the President blasted what he called "illegal leaks" coming out of Washington, in his first public comments since his national security adviser quit.

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?

— Donald J. Trump (@realDonaldTrump) February 14, 2017

Meanwhile, pressure is mounting on North Korea after it carried out a ballistic missile test on Saturday, which was been condemned by the international community.

The UN Security Council has warned of "further significant measures" if Pyongyang, which is already facing a string of sanctions, does not stop nuclear and missile testing.

The Foreign Office summoned North Korea's ambassador to London, Choe II, who was told by British officials that his country's actions "threaten international peace and security".

North Korea tested the ballistic missile while Mr Trump was at his Florida resort hosting Japanese prime minister Shinzo Abe.