He was said to believe they may have been witness to events and discussions that he is investigating.

That included Mr Trump's decision to sack James Comey as FBI Director, and the handling of a controversy involving former national security adviser Michael Flynn and his contacts with the Russian ambassador Sergey Kislyak.

The other three aides Mr Mueller may want to speak to were White House lawyer Don McGahn, one of his deputies James Burnham, and a White House spokesman Josh Raffel, the Washington Post reported.

Mr Mueller's investigation is trying to establish whether anyone involved with the Trump campaign engaged in coordination with Russia in an attempt to influence last year's presidential election.

He is also looking into whether there was any attempt to obstruct justice in relation to the sacking of Mr Comey