A US federal appeals court has rejected President Donald Trump's attempt to reinstate his ban on citizens from seven mainly Muslim countries.

The 9th US Circuit Court of Appeals said it would not block a ruling by a Seattle court that halted the order.

Mr Trump responded with an angry tweet saying national security was at risk and there would be a legal challenge.

It was unclear whether he intended to file an appeal to the Supreme Court or keep fighting the case in Seattle.

In its 3-0 unanimous ruling on Thursday, the appeals court said the government had not proved the terror threat justified reviving the ban.

The ruling means that people from Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen with visas can continue to enter the US, and refugees from around the world, who were also subject to a temporary ban, are no longer blocked either.

However, the ruling does not affect one part of Mr Trump's controversial executive order: a cap of 50,000 refugees to be admitted in the current fiscal year, down from the ceiling of 110,000 established under his predecessor, Barack Obama.

They accepted that the US president had sole discretion to set immigration policy but added that courts could still assess the order's compliance with the constitution.

They discussed whether the ban violated the constitution by targeting a religious group (Muslims) but did not reach a conclusion.

The judges also found "no evidence that any alien from any of the countries named in the order" had committed a terrorist attack in the US.

However, the judges said both sides had made compelling cases: "On the one hand, the public has a powerful interest in national security and in the ability of an elected president to enact policies.

"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."

The appeal judges did not rule on the constitutionality of the order, just on the question of ending the temporary ban.