Washington state's Attorney-General said he would depose Trump administration officials to uncover "what truly motivated" President Donald Trump's executive order on immigration if the case brought against it is taken to the Supreme Court.

Key points: Attorney-General's comments follow those of a senior adviser who said Trump will win eventually

Attorney-General's comments follow those of a senior adviser who said Trump will win eventually Senior White House adviser said barring US entry is the "apex of presidential authority"

Senior White House adviser said barring US entry is the "apex of presidential authority" Mr Trump has suggested that he may issue a new executive order over fighting the courts

Attorney-General Bob Ferguson said the documents and emails authored by administration officials may contain evidence that the order was an unconstitutional attempt to ban Muslims from entering the United States, and pledged on ABC America's This Week program that he would use "every tool" at his disposal to bring those attempts to light.

Mr Ferguson's comments came in response to those of White House senior policy adviser Stephen Miller, who over the weekend repeatedly called the federal appeals court's decision to block the order a "judicial usurpation of power" and said the world would see "the president's powers here are beyond question".

"It has been an important reminder to all Americans that we have a judiciary that has taken far too much power and become in many cases a supreme branch of government," Mr Miller told CBS' Face The Nation program.

"One unelected judge in Seattle cannot remake laws for the entire country, I mean this is just crazy.

"The end result of this though is that our opponents, the media, and the whole world will soon see — as we begin to take further actions — that the powers of the president to protect our country are very substantial and will not be questioned."

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The Trump administration has faced multiple legal setbacks to its travel ban issued on January 27, and Mr Trump said he may issue a new executive order rather than go through lengthy court challenges.

In his comments over the weekend, Mr Miller was referring to immigration laws on which the executive order is based that gives Mr Trump broad powers to restrict who enters the country on national security grounds.

However, the same law forbids discrimination on race, sex, nationality or place of birth or residence — the case also could involve First Amendment protections involving religion.

Restricting US entry 'the apex of presidential authority'

Mr Trump's top brass: White House Chief Strategist Stephen Bannon (L), Chief of Staff Reince Priebus (R) and Senior Policy Adviser Stephen Miller (Back). ( Reuters: Carlos Barria, file )

The executive order Mr Trump issued banned entry into the United States to refugees and citizens of seven Muslim-majority countries, triggering nationwide protests and legal challenges.

A week later, a federal judge in Seattle issued a temporary restraining order that put the president's travel ban on hold, eliciting a barrage of angry Twitter messages from Mr Trump.

The judge's suspension was then upheld by a three-judge panel of the 9th US Circuit Court of Appeals in San Francisco on Thursday.

Mr Miller, appearing on several television news shows, criticised the court and its ruling.

"The 9th Circuit has a long history of being overturned and the 9th Circuit has a long history of overreaching," he said on the Fox News Sunday program.

"This is a judicial usurpation of power."

The powers to restrict entry into the United States "represent the very apex of presidential authority", he added.

Reuters