Michael Flynn, Donald Trump’s national security adviser resigned over his contacts with Russia, the White House announced.

Mr Flynn allegedly discussed US sanctions with the Russian ambassador to the US, Sergey Kislyak, before Mr Trump took office. 

Reports have surfaced that the Justice Department warned the Trump administration last month that Flynn misled administration officials regarding his communications with the Russian ambassador to the US, Sergey Kislyak, and was potentially vulnerable to blackmail by the Russians, a person familiar with the matter told CNN.

A White House official also confirmed the Justice Department warning.

The concern was raised after Flynn claimed that he did not discuss with the Russian ambassador the sanctions being imposed by the former President Barack Obama’s administration in retaliation for Russia’s interference in the 2016 presidential election.

However, at the time, Flynn was not yet in government.

The message was delivered by then-Acting Attorney General Sally Yates. Other top intelligence officials, including James Clapper and John Brennan, were in agreement the White House should be alerted about the concerns.

White House press secretary Sean Spicer issued a statement on Monday saying President Trump was “evaluating the situation” around Flynn, who is in hot water after possibly misleading Vice President Mike Pence.

“He’s (Trump) speaking to the Vice President relative to the conversation the vice president had with General Flynn, and also speaking to various other people about what he considers the single most important subject there is: our national security.”

The noncommittal statement came shortly after Kellyanne Conway, the counsellor to the President, said that Trump has “full confidence” in Flynn.

“General Flynn does enjoy the full confidence of the President,” Conway told MSNBC News.

She later declined to detail how much the President knew about the issue and when he knew it, deeming those conversation private.

Many inside the Trump administration are concerned with the fact that the national security adviser could have misled senior members of the White House, including Pence, who went on television and denied that Flynn spoke about sanctions with Kislyak.

House Minority Leader Nancy Pelosi has called the reports “proof he should not be entrusted with our national security”, The Hill magazine reported.

Democrats are now demanding that the administration release the transcripts of the calls, arguing that Flynn violated an obscure and likely unenforceable 1799 law prohibiting private citizens from engaging in foreign policy.

The White House confirmed on Monday that Flynn had apologised to Pence.

But the decision of whether to demand the resignation of a national security adviser ultimately rests with the President, the White House added.