As President Donald Trump begins interviewing candidates for the job of National Security Adviser left vacant by the sensational firing of Michael Flynn, US senators have formally requested the preservation of materials relevant to a probe on Russian interference in the 2016 election.

While the inquiry remains in its early stages, it threatens to uncover material about the Trump campaign’s contact with Russia that could be damaging for the new President and his administration.

There is strong public interest in uncovering any links between Russia and the Trump organisation after it was revealed that Russia hacked the Democratic Party during the election. Suspicions heightened last week when The New York Times reported that Trump staffers had repeated contact with Russia ahead of polling day.

The moves follow the departure of Mr Trump’s first choice for National Security Adviser, Mr Flynn, who was sacked only weeks into the role after it was revealed he lied to Vice President Mike Pence about discussing sanctions with the Russian ambassador before Mr Trump was sworn into office.

Late last week, Senate Democrats wrote to the White House and law enforcement agencies seeking assurances that they were preserving all materials related to contacts individuals associated with Mr Trump had with Russians.

Congressional staffers have said that they were not aware of any evidence that materials were not being preserved, but Senate Democratic leader Chuck Schumer of New York spoke about the issue on the Senate floor.

“There is real concern that some in the administration may try to cover up its ties to Russia by deleting emails, texts and other records that could shine a light on those connections.”

While the investigation gets underway, Mr Trump will interview acting national security adviser Keith Kellogg and three other candidates to fill the vacancy left by the firing of Mr Flynn.

Michael Flynn was sacked as National Security Adviser last week. Photo: Getty

Former US ambassador to the United Nations John Bolton, Lieutenant General HR McMaster and Lieutenant General Robert Caslen are also being interviewed for the top national security position, White House spokesman Sean Spicer told reporters at Mr Trump’s Mar-a-Lago resort in Florida on Saturday.

Mr Trump, who has been searching for a new national security adviser for about a week, could add a couple more candidates to the list, Mr Spicer said.

Retired general and former CIA chief David Petraeus is no longer a candidate, Mr Spicer said.

Finding a replacement has been a challenge for Mr Trump, with reports that the new President was struggling to find people who had not openly criticised him to fill high-ranking roles.

The New York Times reports that the Trump administration had rejected long lists of candidates for key positions because they had disparaged him during the election campaign.

– with AP, RAW