US President Donald Trump has sacked his Attorney-General, Jeff Sessions, one day after the Republicans lost control of Congress in the midterm elections.

Key points: Mr Sessions said in his resignation letter that it came at Mr Trump's request

Mr Sessions said in his resignation letter that it came at Mr Trump's request The US leader criticised Mr Sessions for recusing himself from the Russia probe

The US leader criticised Mr Sessions for recusing himself from the Russia probe The new Acting Attorney-General once said funding for the Russia probe could be cut to make the investigation "grind to almost a halt"

The move came after Mr Trump repeatedly criticised Mr Sessions for recusing himself from an investigation into Russia's role in the 2016 presidential race.

Mr Sessions announced his resignation in a letter to Mr Trump, saying the resignation came at "your request".

The 71-year-old former US senator was informed on Wednesday morning (local time) by White House chief of staff John Kelly in a phone call that it was time to go, an aide who spoke on condition of anonymity told Reuters.

It was the first of what could be a string of high-profile exits as Mr Trump reshapes his team ahead of his own 2020 re-election effort.

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Mr Sessions' departure was widely expected in the wake of the midterms, which saw Republicans retain their majority in the Senate but lose control of the House of Representatives.

Acting Attorney-General Matthew Whitaker will oversee all matters under the Justice Department's purview, including the Russia probe that had been supervised by Deputy Attorney-General Rod Rosenstein, a department spokeswoman said on Wednesday.

In a tweet, Mr Trump thanked Mr Sessions for his service, and wished him well.

He said a permanent replacement would be nominated at a later date.

Mr Sessions walked out the Justice Department for the last time on Wednesday evening to applause from more than 150 employees who gathered in a courtyard.

He appeared emotional as he left and said, "Thank you" and "God bless," before hopping into a waiting car.

He also shook hands with Mr Whitaker, who told Mr Sessions: "It's been an honour, sir."

In a statement, Mr Whitaker called Mr Sessions a man of integrity "who has served this nation well" and a "dedicated public servant for over 40 years".

He also said he was honoured that Mr Trump has confidence in his ability to lead the Justice Department.

'Blatant attempt' to end Russia probe

Matthew Whitaker has suggested that an acting attorney-general could cut Mr Mueller's budget. ( AP: Charlie Neibergall, file )

Democrats have criticised the decision to fire Mr Sessions, asserting it was part of a plan that could see the extensive Russia probe wound back.

Senate Minority Leader Chuck Schumer said it was "paramount" that the investigation be protected by President Donald Trump's new Attorney-General.

Senator Schumer said he found the timing of Mr Sessions' departure "very suspect."

Sorry, this video has expired Chuck Schumer suggests Jeff Sessions resignation is suspicious

The New York Democrat said it would spark a "constitutional crisis" if Mr Trump forced out Mr Sessions as a "prelude" to ending or limiting special counsel Robert Mueller's investigation.

House Democratic leader Nancy Pelosi said she saw Mr Sessions' sacking as a "blatant attempt" to undermine the probe, which Mr Trump has frequently labelled a "witch-hunt".

Ms Pelosi called on Mr Whitaker to recuse himself from any involvement in the probe, citing what she called his "record of threats to undermine and weaken" it.

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Mr Whitaker once opined about a situation in which Mr Trump could fire Mr Sessions, and then appoint an acting attorney-general who could stifle the funding of Mr Mueller's probe.

"I could see a scenario where Jeff Sessions is replaced with a recess appointment," Mr Whitaker told CNN in July last year.

"That attorney-general doesn't fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to almost a halt."

Asked if that would be to dwindle the special counsel's resources, Mr Whitaker responded, "Right."

In an op-ed for CNN, Mr Whitaker wrote: "Mueller has come up to a red line in the Russia 2016 election-meddling investigation that he is dangerously close to crossing."

Sorry, this video has expired Donald Trump clashes with CNN's Jim Acosta in a fiery press conference.

If Mr Sessions' departure was an "opening move" by Mr Trump to meddle in the Mueller investigation, "the President must be held accountable", the second-ranking House Democrat, Steny Hoyer, said in a statement on Tuesday.

The announcement of Mr Sessions' resignation came soon after Mr Trump held an at times raucous press conference at the White House, where he slammed fellow Republicans and the media for his party's losses in yesterday's midterms.

Mr Trump also got into a fiery exchange with CNN reporter Jim Acosta, after he asked the US leader about his rhetoric on immigration and the Russia probe.

Bitter relationship after Russia recusal

Never in modern history has a president attacked a Cabinet member as harshly as Mr Trump did Mr Sessions. ( Reuters )

Mr Sessions departs as the nation's top law enforcement officer while special counsel Robert Mueller, operating under the auspices of the Justice Department, pursues a wide-ranging investigation into potential collusion between the Trump presidential campaign and Moscow.

The probe has already yielded a series of criminal charges against several of Mr Trump's associates and has dogged his presidency.

Never in modern history has a president attacked a Cabinet member as frequently and harshly in public as Mr Trump did Mr Sessions following his decision to recuse himself from the Russia investigation.

The President has often used Twitter to air his frustrations with Mr Sessions. In September, he criticised him for indicting two Republican congressmen ahead of the midterm elections.

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Mr Sessions had been one of the first members of Congress to back his presidential campaign in 2015.

Rejecting White House entreaties not to do so, Mr Sessions stepped aside from overseeing the FBI probe, citing news reports of previously undisclosed meetings he had with Russia's ambassador to Washington as his reason.

Deputy Attorney-General Rod Rosenstein took over supervision of the Russia investigation and in May 2017 appointed Mr Mueller as the Justice Department's special counsel, taking over the FBI's Russia probe after Mr Trump fired FBI director James Comey.

Mr Rosenstein also has faced criticism from Mr Trump.

A permanent replacement for Mr Sessions must be confirmed by the US Senate, which Mr Trump's Republicans will continue to control as a result of the midterm elections.

ABC/Wires