US Attorney-General Jeff Sessions has abruptly asked the remaining 46 chief federal prosecutors left over from the Obama administration to resign, including Manhattan US Attorney Preet Bharara, who had been asked to stay on in November by then President-elect Donald Trump.

Although US attorneys are political appointees, and the request from Trump's Justice Department is part of a routine process, the move came as a surprise to most. Not every new administration replaces all US attorneys at once.

A Justice Department spokeswoman confirmed the resignation requests included Bharara, whose office handles some of the most critical business and criminal cases passing through the federal judicial system.

Bharara met with Trump in Trump Tower on November 30. After, Bharara told reporters the two had a "good meeting" and he had agreed to stay on.

On Friday, Bharara was unsure where he stood because he did not know if the person who contacted him about resigning was aware that Trump had asked him to remain in office, according to a source familiar with the matter.

It was not immediately clear if all resignations would ultimately be accepted.

A Justice Department spokesman said on Friday Trump had called Obama-appointee Dana Boente, acting US deputy attorney-general, to decline his resignation.

Trump also called Maryland US Attorney Rod Rosenstein, his pick to take over as deputy attorney-general, to keep him in his post, the spokesman said.

Bharara, appointed by Democratic President Barack Obama in 2009, has pursued an aggressive push against corruption in state and city politics and is known for his prosecution of white-collar criminal cases. He also has been overseeing a federal probe into New York City Mayor Bill de Blasio's fundraising.