The White House was warned about national security adviser Michael Flynn's vulnerability to blackmail after he allegedly tried to cover up the nature of his contacts with Russia, according to reports.

According to a report by the Associated Press, the Trump administration was aware "for weeks" of a justice department warning about Flynn's Russia contacts.

Flynn resigned on 13 February, after reports emerged that he had lied about secret communications with Russia in which he had discussed sanctions imposed by the Obama administration, and misled Vice President Mike Pence about the content of the discussions.

In his letter of resignation, Mr Flynn said he had "inadvertently briefed the vice-president-elect and others with incomplete information regarding my phone calls with the Russian ambassador".

A White House statement said Lt Gen Joseph Keith Kellogg had been appointed as interim replacement for the post.

Flynn's actions break US laws prohibiting private citizens conducting diplomacy. Flynn's phone call with Russia's ambassador to the US, Sergei Kislyak, were made before Trump's victory.

Flynn initially denied sanctions had been discussed, and Pence repeated the denial on his behalf and defended Flynn.

The Washington Post reported that former Acting Attorney General Sally Yates told the White House last month that Flynn "had put himself in a compromising position" by speaking to the Russians before he was authorised, and that the Justice Department had learned he had misled Pence. This left Flynn open to Russian blackmail attempts, according to the report.

Yates was sacked by Trump when she opposed his ban banning refugees and those from seven Muslim-majority countries entering the US.