WASHINGTON - In an escalating crisis for his 10-day-old administration, President Donald Trump fired Acting Attorney General Sally Yates after she refused to defend his controversial ban on refugee from entering the United States in court.

The president declared in a statement that Ms Yates, who had served as deputy attorney general under President Barack Obama, had betrayed the administration by announcing that Justice Department lawyers would not defend Trump’s order against legal challenges. The ouster came just hours after Ms Yates said she would not defend in court the president’s executive order suspending immigration from seven majority-Muslim countries.

"Ms Yates is an Obama administration appointee who is weak on borders and very weak on illegal immigration," a statement from the White House said. Trump appointed Dana Boente, the US attorney for the Eastern District of Virginia, to serve as acting attorney general until his nominee, Senator Jeff Sessions, can be confirmed by the Senate.

The Senate Judiciary Committee has scheduled a Tuesday vote on Sessions, who has closely advised Trump on immigration matters. Boente was immediately sworn in at 9 pm Monday (7 am PST Tuesday). "I will defend and enforce the laws of our country to ensure that our people and our nation are protected,” Boente said in a statement issued through the White House.

The stunning series of events that rolled late into Monday evening drew immediate comparisons to the so-called Saturday Night Massacre of 1973, when then Attorney General Elliot Richardson chose to resign rather than obey President Richard Nixon’s order to fire Watergate special prosecutor Archibald Cox.

In a bombshell letter to Justice Department attorneys, Yates said “I am responsible for ensuring that the positions we take in court remain consistent with this institution’s solemn obligation to always seek justice and stand for what is right.”

“At present, I am not convinced that the defence of the executive order is consistent with these responsibilities nor am I convinced that the executive order is lawful.

“Consequently, for as long as I am the acting attorney general, the Department of Justice will not present arguments in defence of the executive order, unless and until I become convinced that it is appropriate to do so," wrote Yates, a holdover from the Obama administration.

Trump responded to Yates via Twitter, characterising the action as a "political" move to block his agenda. "Democrats are delaying my cabinet picks for purely political reasons,” Trump tweeted. "They have nothing going but to obstruct. Now have an Obama AG."

Senate Democrats have objected to Sessions’ nomination, and Minority Leader Charles Schumer, a Democrat, said on CNN that Sessions should have to disclose his opinion of Trump’s immigration order before the Senate votes on his nomination.

Trump adviser Stephen Miller told MSNBC that Yates' decision is “a further demonstration of how politicised our legal system has become,” so much so that “you have people refusing to enforce our laws.” Miller said the president clearly has the legal authority to bar people from entry into the country for national security reasons. But former Obama Attorney General Eric Holder tweeted late Monday that Yates' "judgement should be trusted."