President Donald Trump ban on refugees and travellers from six Muslim-majority countries; US Appeals Court.

Grandparents and other relatives of people living in the US cannot be barred from entering under President Donald Trump's travel ban, a judge has ruled.

Donald Trump's administration vowed Friday to appeal to the Supreme Court after a federal judge dealt a fresh setback to the president's ban on travellers from six mainly Muslim countries and refugees as unfairly excluding the grandparents and grandchildren of people living in the United States.

Attorney General Jeff Sessions, in a strongly-worded statement announcing the appeal, accused the Hawaii court of having "undermined national security, delayed necessary action, created confusion and violated a proper respect for separation of powers."

He charged the lower court had "improperly substituted its policy preferences for the national security judgments of the executive branch in a time of grave threats, defying both the lawful prerogatives of the executive branch and the directive of the Supreme Court."

It was unclear how quickly the Supreme Court now in summer recess but able to act on emergency motions might be able to respond, and when or if the expanded terms set by the Hawaii judge might take effect. If they do, thousands of potential travellers could be affected.

The travel ban, which the administration insists is necessary to keep violent extremists out of the country, has faced a series of judicial roadblocks from lower courts.

But the administration gained partial satisfaction in June when the Supreme Court ruled that it could proceed with its plan to prohibit the entry of some people from countries deemed dangerous, while still allowing visits by people with "a credible claim of a bona fide relationship with a person or entity in the United States."

The Supreme Court's ruling allowed a 90-day ban on visitors from Iran, Libya, Somalia, Sudan, Syria and Yemen, and a 120-day ban on refugees to come into force, with exceptions for those people with "bona fide" relationships.

That June 29 ruling, which capped months of legal wrangling, left unclear the question of just who had such a "credible claim." The Trump administration then provided a list defining that category as including parents, spouses, children, sons- and daughters-in-law, siblings and step- or half-siblings.

But the Hawaii judge, Federal Judge Derrick Watson ruled that the administration's criteria unfairly exclude grandparents and grandchildren.

The decision late Thursday thus expanded the exceptions to the ban, ordering that the list include grandparents, grandchildren, brothers-in-law, sisters-in-law, aunts, uncles, nieces, nephews and cousins of people in the United States.

- Common sense -

Watson said the government's "narrowly defined list finds no support in the careful language of the Supreme Court or even in the immigration statutes on which the government relies."

"Common sense, for instance, dictates that close family members is defined to include grandparents," he wrote.

"Indeed, grandparents are the epitome of close family members. The government's definition excludes them. That simply cannot be."

The judge also ruled that refugees who have assurances of a placement by an agency in the United States should be exempt.

White House Homeland Security Adviser Tom Bossert told journalists the ruling seemed "fairly broad," adding that he would be troubled "if it was as broad as reported."

The original ban, announced days after Trump became president on January 20, was successfully challenged in lower courts on the grounds that it overstepped Trump's presidential authority and discriminated against Muslims in violation of the US constitution. A revised version also did not pass legal muster.

Judges in lower courts had cited Trump's repeated statements during the presidential campaign that he intended to ban Muslims from entering the United States.

People have suffered enough

Douglas Chin, attorney general for the state of Hawaii, which filed the lawsuit against the Trump administration, welcomed the Thursday ruling.

"The federal court today makes clear that the US government may not ignore the scope of the partial travel ban as it sees fit," said Chin.

"Family members have been separated and real people have suffered enough. Courts have found that this executive order has no basis in stopping terrorism and is just a pretext for illegal and unconstitutional discrimination."