Robert Mueller, the special counsel investigating allegations of Russian interference in last year's presidential election, has asked for documents on Michael Flynn, the president's former national security adviser.

The request, which was reported in the New York Times, is the first direct request from Mr Mueller's team for White House material.

Details emerged a day after reports that Mr Mueller has convened a grand jury in Washington to participate in the probe, signalling the possibility that the inquiry could result in criminal charges.

While Mr Mueller steps up his investigation, Donald Trump has again angrily denied that his campaign team colluded with the Kremlin.

At a campaign rally in West Virginia on Friday, he dismissed the allegations and investigation as "fake news".