Donald Trump fires US Attorney General Jeff Sessions

United States Attorney General Jeff Sessions has been fired by Donald Trump on Wednesday after enduring more than a year of bitter criticism from the president over his decision to step aside from the Russia investigation that has dogged the White House.

"We thank Attorney General Jeff Sessions for his service, and wish him well!" Trump tweeted on Wednesday.

In announcing the resignation Trump right away named Sessions' chief of staff Matthew Whitaker as acting attorney general.

The move cast into doubt the ability of Special Counsel Robert Mueller who had until now been insulated from White House interference to complete the Russia probe, and drew stern warnings from Democrats for the president to stay clear.

Sessions' departure came the day after a midterm vote that saw Republicans lose control of the House of Representatives but boost their Senate majority, and Trump's swift move was seen as a sign of how he confident he was feeling after the polls.

That set off immediate alarm bells: Whitaker has been overtly critical of the broad scope granted to Mueller's team to probe beyond allegations Trump's campaign colluded with Russia in 2016, into other ties between Trump, his family and aides, and Russia -- an investigation the president calls a "witch hunt".

In an op-ed in August last year he publicly urged Deputy Attorney General Rod Rosenstein -- who oversees the probe -- to "limit the scope of his investigation to the four corners of the order appointing him special counsel."

As acting attorney general, Whitaker now has the power to wrest oversight away from Rosenstein, and take charge himself.

Democrats were outraged by the attorney general's removal, with the Democratic National Committee noting that the appointee has not been confirmed for the role by the US Senate as required.

The Democrats' Senate leader Chuck Schumer said protecting the Mueller investigation was "paramount" in light of the move.

Senator Mark Warner, senior Democrat on the Senate Intelligence Committee, was among those who sounded a warning to Trump.

"No one is above the law and any effort to interfere with the special counsel's investigation would be a gross abuse of power by the president," he said.

Democratic Congressman Jerry Nadler, who is expected to take charge of the House Judiciary Committee in the wake of Tuesday's elections, was more direct:

"There is no mistaking what this means, and what is at stake: this is a constitutionally perilous moment for our country and for the president."

Sessions was the first casualty of a cabinet shakeup that had been expected from Trump following the midterm elections.

But his departure had been anticipated since early this year, after he endured withering and repeated criticism from the president over the legally troubled ban on Muslim travellers Trump sought when he came into office, and over the Mueller probe.

"At your request, I am submitting my resignation," Sessions said in the first line of a letter addressed to Trump, released by the Department of Justice.