A former acting US attorney-general has said she told the White House their pick for national security adviser "essentially could be blackmailed" by the Russians.

Sally Yates told a Senate subcommittee she had two meeting and a phone call with Trump administration officials about reports that retired Lieutenant General Michael Flynn had misrepresented his contacts with Russia's ambassador to the US.

She said she told White counsel Don McGahn that the Russians knew Mr Flynn had misled US Vice President Mike Pence and others.

Before the hearing, it emerged that Barack Obama urged Donald Trump not to hire Mr Flynn after he won the election.

Mr Flynn was dismissed as defence intelligence chief by the Obama administration before he became became a Trump supporter.

Mr Trump tweeted that it was Mr Obama's team who gave the future national security adviser the "highest security clearance".

General Flynn was given the highest security clearance by the Obama Administration - but the Fake News seldom likes talking about that.

— Donald J. Trump (@realDonaldTrump) May 8, 2017

He also tweeted: "Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel."

Ms Yates was fired by the Trump administration in January after she refused to defend travel restrictions on people from seven Muslim-majority countries.

Mr Flynn was forced to step down after just 24 days as national security adviser after multiple outlets reported that he had misled Vice President Pence about conversations he had with Russia’s ambassador to the US, Sergey Kislyak, before Trump's inauguration.