The US Senate confirmed Jeff Sessions as the next attorney general, following a bitter debate in the chamber that saw Republicans formally rebuke Senator Elizabeth Warren for the manner in which she criticised her colleague from Alabama.

Sessions, a four-term US senator, was the first senator to endorse Trump in February 2016, and his conservative, populist views have shaped many of the Administration's early policies, including on immigration.

The vote, 52-47 in favour of confirmation, ran largely down party lines.

Republicans accused Democrats of seeking to undercut Trump by attempting to derail his Cabinet choices. "It's no secret that our Democrat colleagues don't like the new president and are doing what they can to undermine the new administration," said Senator Chuck Grassley, the Judiciary Committee chairman.

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He expressed disappointment in colleagues who, he said, suggested Sessions won't be able to put aside his policy preferences and enforce the law. "This is especially troubling after he specifically committed to us during his confirmation hearing that, if he's confirmed, he will follow the law, regardless of whether he supported the statute as a policy matter," Grassley said.

Leading Democrats have argued that Trump's criticisms of the federal courts over his immigration order makes the need for an attorney general who will be willing to disagree with the President even more urgent.

"What we've seen is a president who belittles judges when they don't agree with him," said Senate Minority Leader Chuck Schumer.

"What we've seen is a president who is willing to shake the roots of the Constitution and a fundamental premise - no religious test - that's embodied in our Constitution within his first few weeks in office," Schumer said.

"We certainly need an attorney general who will stand up to that president . . .. But [Sessions] is not, if you can say one thing about him, he's not independent of Donald Trump."

Sessions, 70, advanced out of the judiciary committee last week after a vote along party lines. The hearing took place after then-acting Attorney General, Sally Yates, an Obama Administration holdover, had ordered the department's lawyers not to defend Trump's immigration order on grounds that she was not convinced it was lawful. Within hours, Trump fired her.

In his confirmation hearing last month, Sessions repeatedly vowed to put the law above his personal views. He said he would abide by the Supreme Court decision underpinning abortion rights and a court ruling legalising same-sex marriage. He said he understood that the waterboarding of terrorism suspects to elicit information is "absolutely improper and illegal" and, though he voted against it, he would uphold a law banning the government's bulk collection of phone records.

He also declared that he would recuse himself from Justice Department probe of Hillary Clinton's email practices or her family's charitable foundation, mindful that his previous comments "could place my objectivity in question".

But he has repeatedly declined to say whether he would recuse himself from any investigation involving Trump associates and possible links to Russia's interference in the presidential election, saying he would seek the recommendations of department ethics officials and "value them significantly" in making a decision.