U.S President Donald Trump fired the acting attorney general, a holdover from the Obama administration, after she ordered Justice Department attorneys not to defend his controversial immigration orders.

In a sharply worded statement, the White House called Sally Yates "weak on borders and very weak on illegal immigration" and also criticized Democrats for not yet confirming the appointment of attorney general-designate Jeff Sessions.

"The acting attorney general, Sally Yates, has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States," the White House said in a statement.

"This order was approved as to form and legality by the Department of Justice Office of Legal Counsel," it said.

"Tonight, President Trump relieved Ms Yates of her duties."

Federal prosecutor Dana Boente will serve as acting attorney general "until Senator Jeff Sessions is finally confirmed by the Senate, where he is being wrongly held up by Democrat senators for strictly political reasons," it said.

With Trump's White House facing multiple lawsuits and worldwide contempt over an order banning migrants from seven Muslim nations, Yates had whipped the rug from under her boss in a defiant and damaging parting shot.

In a memo to Department of Justice staff, Yates -- a career government lawyer promoted by Barack Obama -- expressed doubts about the legality and morality of Trump's decree, which has prompted mass protests.

"My responsibility is to ensure that the position of the Department of Justice is not only legally defensible, but is informed by our best view of what the law is," Yates wrote.

"I am not convinced that the defense of the executive order is consistent with these responsibilities nor am I convinced that the executive order is lawful," she added.

"For as long as I am the acting attorney general, the Department of Justice will not present arguments in defense of the executive order, unless and until I become convinced that it is appropriate to do so."

Yates's directive means that the U.S government, at least for now, has no authorized courtroom representation in the lawsuits.

It was a remarkable act of defiance against a tough-talking president who has showed little sign of brooking insubordination.