A US court on Thursday unanimously refused to reinstate Donald Trump's ban on refugees and nationals from seven Muslim-majority countries, dealing the new president and his controversial law-and-order agenda a major defeat.

The ruling from the federal appeals court in San Francisco on Trump's executive order, issued on January 27 with no prior warning and suspended by a lower court a week later, capped a turbulent first three weeks of his presidency.

A defiant Trump quickly pledged to battle on, tweeting within minutes of the decision: “SEE YOU IN COURT, THE SECURITY OF OUR NATION IS AT STAKE!”

The Justice Department had asked the Ninth Circuit Court of Appeals to restore the measure on an emergency basis, but the three-judge panel instead maintained the suspension ordered by a federal judge in Seattle.

“We hold that the government has not shown a likelihood of success on the merits of its appeal, nor has it shown that failure to enter a stay would cause irreparable injury,” the judges ruled.

Trump's decree summarily denied entry to all refugees for 120 days, and travelers from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen for 90 days.

Refugees from Syria were blocked indefinitely.

The new Republican administration argued the ban was needed to prevent the militant Islamic State (IS) and Al Qaeda fighters migrating from reaching US soil, but it sparked travel chaos and was roundly rejected by immigration advocacy groups.

Critics say the measure targeted Muslims in violation of US law.

Now, the case could end up in the Supreme Court.

Court's logic

The court in San Francisco said aspects of the public interest favoured both sides, highlighting the “massive attention” the case had drawn.

“On the one hand, the public has a powerful interest in national security and in the ability of an elected president to enact policies,” the ruling said.

“And on the other, the public also has an interest in free flow of travel, in avoiding separation of families, and in freedom from discrimination.”

While acknowledging that the Seattle judge's ruling “may have been overbroad in some respects”, the panel said it was not their “role to try, in effect, to rewrite the executive order”.

“The government has pointed to no evidence that any alien from any of the countries named in the order has perpetrated a terrorist attack in the United States,” the court said.

'No one is above the law'

Civil rights campaigners and state officials applauded the decision, vowing to fight on until the executive order was permanently scrapped. For now, it means that travellers with valid visas can continue to enter the country.

Washington Governor Jay Inslee, whose administration sued for the measure to be blocked, hailed a victory for his state and the country, arguing that the ruling showed “no one is above the law, not even the president”.

Human Rights Watch senior researcher Grace Meng called the decision “an important declaration of judicial independence, which is crucial for checking harmful overreach by the president”.

Trump had blasted the original suspension and decried the politicisation of the federal courts in a series of fiery tweets and public statements.

Republican lawmakers jumped to Trump's defence, with Senator Tom Cotton calling the ruling “misguided”, while Democrats predictably hailed it.

“Pres Trump ought to see the writing on the wall, abandon proposal, roll up his sleeves & come up w/ a real, bipartisan plan to keep us safe,” tweeted Senate Democratic minority leader Chuck Schumer.

Trump's election rival Hillary Clinton tweeted simply: “3-0”.

'New era of justice'

Ahead of the ruling, and with tensions high between the executive and the judiciary, Trump defended his hardline policies and declared a “new era of justice” in America as he swore in Attorney General Jeff Sessions.

“We face the menace of rising crime and the threat of deadly terror,” he said, doubling down on his dystopian vision of America.

“A new era of justice begins and it begins right now.”

Trump's tough talk belies a political and legislative agenda that has been beset by missteps and legal challenges.

Even his own Supreme Court nominee, Neil Gorsuch, described the president's comments about the judiciary as “disheartening” and “demoralizing”.

Popular support

Though Trump's message has been criticised by experts, it appears to be resonating with supporters. The billionaire won the election last November with 46 per cent of the popular vote, and the RealClearPolitics average of polls shows his job approval at about that level, with the split largely along Republican-Democratic lines.

Trump on Wednesday trumpeted a Morning Consult-Politico poll showing 55pc voter approval for his immigration ban, although earlier polls — dismissed by the president as “fake news” — have shown majority opposition.

His administration has 14 days to file a petition for reconsideration, either by the same panel or “en banc” — meaning by every judge on the court.

Another option would be to ask the Supreme Court to review the case, although some analysts have argued that this path poses the possibility of an embarrassing defeat, given the unanimity of the San Francisco panel, which included a Republican-appointed judge.