Sally Yates, Acting Attorney General, has been fired by US President Donald Trump for betraying the Department of Justice, a White House statement read.

US President Donald Trump has fired the federal government's top lawyer Sally Yates after she took the extraordinarily rare step of defying the White House and saying the Justice Department would not defend his new travel restrictions targeting seven Muslim-majority nations.

He has also fired the acting director of US Immigration and Customs Enforcement, Daniel Ragsdale, appointing Thomas D. Holman in his place. There was no explanation behind this change.

In a statement Trump announced the appointment of Dana Boente as acting attorney general in her place, saying Yates had "betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States".

JONATHAN ERNST/REUTERS US President Donald Trump has fired acting US Attorney General Yates.

"Yates is an Obama Administration appointee who is weak on borders and very weak on illegal immigration," he said.

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"It is time to get serious about protecting our country. Calling for tougher vetting for individuals travelling from seven dangerous places is not extreme. It is reasonable and necessary to protect our country."

Boente was sworn in in a hastily arranged ceremony after Trump dismissed his predecessor for instructing Justice Department lawyers to stop defending Trump's travel and refugee ban.

Reporters and news photographers were not invited to witness the ceremony.

Boente has been serving as US Attorney for the Eastern District of Virginia.

He's expected to serve until Senator Jeff Sessions, Trump's pick for the position, is confirmed by the Senate.

Yates told Justice Department lawyers in a letter that they would not defend in court Trump's directive that put a 120-day hold on allowing refugees into the country, an indefinite ban on refugees from Syria and a 90-day bar on citizens from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.

Yates said she did not believe defending the order would be "consistent with this institution's solemn obligation to always seek justice and stand for what is right."

There have been only a handful of instances in US history of top Justice Department officials publicly breaking with the White House.

The most famous example was in 1973, when then-Attorney General Elliot Richardson resigned rather than obey President Richard Nixon's order to fire a special prosecutor investigating the Watergate scandal.

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