US National Security Adviser Michael Flynn has resigned over allegations that he discussed US sanctions with Russia before Donald Trump took office.

Flynn is said to have misled officials about his call with Russia’s ambassador before his own appointment. It is illegal for private citizens to conduct US diplomacy.

Reports earlier said the White House had been warned about the contacts last month and was told Flynn may be vulnerable to Russian blackmail.

The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.

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

Flynn, a retired Army lieutenant general, initially denied having discussed sanctions with Russian Ambassador, Sergei Kislyak, and Vice-President Mike Pence also publicly denied the allegations on his behalf.

But the former security adviser came under further pressure on Monday when details of his phone call emerged in US media, as well as reports that the justice department had warned the White House about him misleading senior officials and being vulnerable to Russian blackmail.

It was gathered that the message was delivered by then-acting attorney general Sally Yates, who was subsequently dismissed by President Trump for opposing his controversial travel ban.

Kremlin spokesman Dmitry Peskov said Russia would not be commenting on the resignation.

“This is the internal affair of the Americans, the internal affair of the Trump administration. It’s nothing to do with us,” he added.

Other Russian lawmakers have spoken out in defence of Flynn, with Alexei Pushkov tweeting that he had been “forced to resign not because of his mistake but because of a full-fledged aggressive campaign. Trump is the next target,” he tweeted.

Meanwhile, the opposition Democratic Party is already taking advantage of the development to further criticize the Trump presidency.

Senior Democrat Adam Schiff said Flynn’s departure would not end questions about any contacts between Donald Trump’s campaign and Russia.

Similarly, two Democrat members of the judiciary and oversight committees, John Conyers and Elijah Cummings, have also demanded a classified briefing to Congress on Michael Flynn by the Justice Department and FBI.

“We in Congress need to know who authorised his actions, permitted them, and continued to let him have access to our most sensitive national security information despite knowing these risks,” their statement said.

Flynn, who was previously fired by Barack Obama as head of the Defense Intelligence Agency, was an ardent supporter of Trump during the campaign.

He encouraged tougher policies on Iran and a softer policy on Russia, but questions were raised about his perceived closeness to Moscow.

Joseph Keith Kellogg, a retired US Lieutenant General has been appointed acting national security adviser, and is far from a newcomer to the Trump team.

He brings more than 30 years’ experience in the army, and served in Vietnam, Cambodia, Panama and the Gulf.

During the Iraq war, he helped manage the coalition authority running the country in 2003 and 2004, before working for a defence contractor.

More recently, he advised President Trump on national security issues during his campaign, and went on to be appointed chief of staff in the new administration’s National Security Council.

Former CIA director retired General David Petraeus and Robert Harward, a former deputy commander of US Central Command, are also under consideration for the post, a White House official has said.