The Justice Department has tapped ex-FBI Director Robert Mueller to serve as Special Counsel in the probe into alleged ties between associates of US President Donald Trump and the Russian government. The White House and Moscow have denied the accusations.

Lawmakers on both sides of the aisle in Congress have called for a special investigator to watch Trump after former FBI head James Comey was fired on May 9. On Wednesday, they got their wish.

"I have accepted this responsibility," Mueller confirmed, "and will discharge it to the best of my ability."

"My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination," Deputy Attorney General Rod Rosenstein said in a statement. "What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command."

Comey had been overseeing an investigation into Trump, which led some observers to charge that the president had obstructed justice by interfering in an ongoing investigation.

House Republican leader Paul Ryan urged Mueller to pursue the probe without assuming any conclusions. "Follow the facts wherever they lead," Ryan said.

Senator Richard Burr, who leads the Senate investigation, said "I think this was a good decision." Mitch McConnell, who was initially skeptical of the need for a special prosecutor, said the Senate's investigation under Burr "will continue."

Jason Chaffetz, chairman of the House Oversight Committee, said Mueller's appointment was a "great selection" that "should be widely accepted."

House Minority Leader Nancy Pelosi applauded Mueller as a "respected public servant of the highest integrity," but noted "Director Mueller will still be in the chain of command under the Trump-appointed leadership of the Justice Department." She said this is a good first step, but it shouldn't be the last, and that an independent commission should be formed.

A White House official told Politico that Trump was unaware a special counsel was being named until about 60 minutes before the announcement. At that point, Rosenstein had already signed the order, Axios reports.

Later on Wednesday, the White House released a statement reiterating there has been no collusion "with any foreign entity."

​On Tuesday, an explosive report emerged suggesting Trump had asked the director to close a probe concerning ex-National Security Adviser Michael Flynn.

US President Donald Trump indicated in a letter that Comey had informed him on three occasions that he was not under investigation by the FBI for illegal links between campaign officials and Russian government operatives. This isn't standard practice for the FBI, however, according to acting director Andrew McCabe.

Trump initially exclaimed he had "accepted the recommendation" of Justice Department officials to fire Comey on May 9. Two days later, the president changed course. Trump was going to fire Comey "regardless" of the DOJ's advice, he claimed last Thursday.