White House staffers have been told to preserve any materials related to Russian intervention in the 2016 presidential election, according to The Associated Press.

The White House counsel's office handed down the order on Tuesday in a memo to aides, the news wire reported.

The move comes after Democrats on the Senate Judiciary Committee asked the White House and law enforcement agencies to keep materials related to Trump associates' alleged ties to Russia.

Separately, the Senate Intelligence Committee last month asked federal agencies to keep materials that tie into its investigation into Russian cyber hacking and other efforts to disrupt the election.

Democrats have feared that the White House could try to hide the extent of its links to Moscow.

"The downfall does not come from the crime, it comes from the cover-up, and that's what we're dealing with now," Senate Minority Whip Dick Durbin (D-Ill.) told Bloomberg on Monday.

Trump has been dogged by allegations of improper ties to Russia during his first weeks in office.

Retired Army Lt. Gen. Michael Flynn resigned as national security adviser last month because he misled Vice President Pence and others about the nature of his conversations with Russia's ambassador to the U.S. before Trump's inauguration.

Some Trump campaign advisers are reportedly under federal investigation over their alleged links to Russian intelligence operatives. They have denied any such ties.

The president himself has dismissed the charges.

"Nobody that I know of," Trump said at a mid-February news conference when asked if anyone who advised his campaign had ties to Russia.

Investigators are reportedly looking into whether anyone tied to Trump colluded with Russia as it hacked emails from Democratic political groups and arranged for them to be published online, an effort U.S. intelligence agencies said was designed to help Trump and hurt Democratic nominee Hillary Clinton.

No proof of collusion has been found.