President Donald Trump said he had a nasty and contentious business relationship with Robert Mueller, and called on the special counsel to disclose such "conflicts of interest" in one of his most direct attacks on the probe into Russian interference in the 2016 election.

Trump didn't say what those conflicts were, but said he turned down Mueller to lead the FBI for a second time before Mueller was appointed special counsel, and that James Comey, whom Trump fired as FBI chief, is Mueller's "close friend."

"Is Robert Mueller ever going to release his conflicts of interest with respect to President Trump, including the fact that we had a very nasty & contentious business relationship," Trump said in a tweet on Sunday, one of many in an active day on Twitter complaining about the investigation and press coverage.

Trump said in a follow-up tweet that Mueller, who led the FBI from 2001 to 2013 and is known to be a lifelong Republican, only appointed as prosecutors "angry" Democrats who have contributed to or previously supported Democrats, and is failing to investigate Democrats.

The Washington Post reported in July 2017, citing two unnamed White House advisers, that Mueller and the Trump National Golf Club in Virginia had a dispute over fees when Mueller resigned as a member in 2011. A spokesman for Mueller said there was no dispute when Mueller, who was FBI director at the time, left the club, the newspaper said.