Donald Trump tried Saturday to invoke a law last used during World War II against Japanese residents to target a Venezuelan gang — only to find his decision blocked by a judge.

The US president had issued an order to deport alleged members of the Venezuelan drug gang Tren de Aragua, arguing he had a right to declare them "alien enemies" under a wartime act.

But rights lawyers went to court and a federal judge ordered the administration not to deport anyone until he has more time to consider the legality of the order.

The Alien Enemies Act of 1798 is a wartime authority that allows a president to detain or deport citizens of an enemy nation, and has been invoked three times.

It was used in the War of 1812, World War I and — most famously — between 1942 and 1946 during World War II to intern around 120,000 Japanese and Japanese-Americans.

Now, Trump, who has promised supporters an aggressive drive to deport thousands of undocumented migrants, is targeting Tren de Aragua.

In a proclamation, the White House declared that the transnational criminal organization is closely linked to the government of Venezuelan President Nicolas Maduro.

"The result is a hybrid criminal state that is perpetrating an invasion of and predatory incursion into the United States," the presidential statement said.

'Extreme measure'

Trump said Tren de Aragua is "conducting irregular warfare against the territory of the United States both directly and at the direction, clandestine or otherwise, of the Maduro regime."

The statement gives Trump's Attorney General Pam Bondi 60 days to enact the ruling making all Tren de Aragua gang members "subject to immediate apprehension, detention and removal."

The detention and expulsion order will apply to all Venezuelan Tren de Aragua members who are over 14 and not naturalized US citizens or lawful permanent residents.

"The Trump administration's intent to use a wartime authority for immigration enforcement is as unprecedented as it is lawless," said Lee Gelernt of the American Civil Liberties Union.

"It may be the administration's most extreme measure yet, and that is saying a lot," said Gelernt, deputy director of the ACLU's Immigrants' Rights Project and lead counsel.

The ACLU and an allied group, Democracy Forward, asked the US District Court in Washington to bar the deportations — arguing that the 1798 act was not intended for use in peacetime.

Judge James Boasberg has yet to rule on the substance of the case, but on Saturday he ordered a 14-day halt to any deportation under the new order at least until he has done so.

Attorney General Bondi said in a statement that the ruling "disregards well-established authority regarding President Trump's power, and it puts the public and law enforcement at risk."

The White House has until Monday to file a motion if they want to overturn the pause, and failing that the next hearing will be on 21 March, the court order said.