In an interview with Fox News aired Friday morning, Trump argued that Mueller, a former FBI director, is "good friends" with James Comey, Mueller's successor at the spy agency whom Trump fired on May 9. Trump later acknowledged he took this step with the Russia probe in mind.

When George W. Bush was president, Mueller and Comey worked together -- Mueller as FBI director and Comey as deputy attorney general.

Trump also said that some of the staffers that Mueller has hired for his investigation "are all Hillary Clinton supporters." US news reports say some of these staffers have made campaign contributions to Democratic candidates.

Asked point blank if Mueller should recuse himself from the Russia probe, Trump said: "Well, he's very, very good friends with Comey, which is very bothersome. But he's also -- we're going to have to see."

Trump added: "I mean we're going to have to see in terms -- look, there has been no obstruction. There has been no collusion. There has been leaking by Comey."

Trump did say, however, that Mueller is an "honorable man."