US Attorney General Merrick Garland on Friday named Jack Smith, a war crimes prosecutor, to serve as special counsel to oversee Justice Department investigations related to Donald Trump including the former president's handling of sensitive documents and efforts to overturn the 2020 election.

Garland's announcement came three days after Trump, a Republican, announced he would run for president again in 2024. Garland said Trump's candidacy, as well as Democratic President Biden's stated intention to run for re-election, made the appointment of a special counsel necessary.

Special counsels are sometimes appointed to investigate politically sensitive cases and they do their jobs with a degree of independence from the Justice Department leadership.

"The pace of the investigations will not pause or flag under my watch," Smith said in a statement. "I will exercise independent judgment and will move the investigations forward expeditiously and thoroughly to whatever outcome the facts and the law dictate."

Smith will oversee the investigation into Trump's handling of government documents after leaving the White House last year and the probe into attempts to interfere with the peaceful transfer of power following the 2020 election, Garland said.

"Appointing a special counsel at this time is the right thing to do," Garland, who was appointed by Biden, told a news conference.

Trump called the special counsel appointment a "rigged deal."

"This horrendous abuse of power is the latest in a long series of witch hunts that started a long time ago," he told a crowd of supporters at a black tie event at his Mar-a-Lago estate in Florida.

Biden did not respond to shouted questions from reporters about the special counsel during his only public appearance of the day. The White House was not involved in the decision to appoint Smith, an official said, speaking on condition of anonymity. (Reuters)