The White House counsel's office has told President Donald Trump's aides to preserve any materials that may be related to Russian interference in the 2016 presidential election, or communication with Russian officials.

The memo was sent to White House staff on Tuesday following a request from the Senate Intelligence Committee. Senate Democrats had also asked the White House and law enforcement agencies to keep all materials involving contacts that Trump's administration, campaign and transition team -- or anyone acting on their behalf -- have had with Russian government officials or their associates.

Trump has been dogged by questions about his advisers' ties to Russia since the campaign. Federal investigators have been looking into possible contacts between Trump advisers and Russia, while congressional committees are investigating Russia's role in political hacking during the campaign.

Congressional staffers have said they are not aware of any evidence that materials related to Russia are not being preserved. But Senate Democratic leader Chuck Schumer of New York said last week: "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. These records are likely to be the subject of executive branch as well as congressional investigations and must be preserved."

The intelligence community has assessed that Russia's hacking of Democratic groups and operatives was carried out to help Trump defeat Democrat Hillary Clinton.

Trump has denied having any knowledge that aides were in touch with Russian intelligence agents during the election.

Last month, FBI Deputy Director Andrew McCabe told White House Chief of Staff Reince Priebus that a New York Times story reporting on alleged contacts between Trump campaign officials and people connected to Russian intelligence was “bulls---."

White House spokesman Sean Spicer later said he was told by CIA Director Mike Pompeo that the agency knew of nothing to contradict McCabe's assessment.

Earlier this year, the FBI interviewed Michael Flynn, then Trump's national security adviser, about his contacts with Russia's ambassador to the U.S. after the election. Flynn was fired after it was revealed that he misled Vice President Mike Pence and other White House officials about the nature of his discussions with the envoy.

Fox News' John Roberts and the Associated Press contributed to this report.