In a two-to-one decision, a panel of judges at the US Court of Appeals in Washington left in place a lower court's injunction preventing the man, a dual US-Saudi national, from being turned over by the US military to the Saudi government.

The American Civil Liberties Union, which has been representing the man, known in court documents only as "John Doe," asked the court to block the transfer, arguing that he has not been charged with a crime and has the right to due process under US laws.

The US government maintains he is an "enemy combatant," which the man denies, and announced on April 17 that it intends to send him to a third country, which court documents implied was Saudi Arabia.

The court's reasoning for its decision to keep the injunction in place remained under seal.

"The appeals court's judgment vindicates due process, limits on executive authority, and the protection of an American's constitutional rights," said ACLU attorney Jonathan Hafetz in a statement.

"The president does not get a blank check to dispose of the liberty of US citizens just because international relations or military actions are involved."

The case is a key test for how the administration of President Donald Trump handles US citizens detained abroad for supporting extremist organizations like the Islamic State group (IS).

The man is the only known US citizen held as an alleged enemy combatant from the battlefields of Iraq and Syria.

Between 100 to 200 US nationals traveled to Syria and Iraq after 2010 to work and fight in IS's ranks, according to various estimates.

It is not clear why the government refuses to hand him over to the US justice system, as other Americans accused of terrorism have been.

But analysts think the Trump administration wants to avoid the fundamental question of whether an American caught fighting for IS has any rights.

The Justice Department on Monday said it would was reviewing the decision to see what steps it will take next.

"This individual's alleged activities with (IS) implicate numerous national security, law enforcement, international relations and foreign policy concerns," said spokeswoman Kerri Kupec.

"Both domestic and international law confer on the US military broad discretion over battlefield operation, including the transfer of individuals captured on overseas battlefields."