A Trump-appointed federal judge has ruled that the US president cannot use the Alien Enemies Act to deport Venezuelan migrants.

US District Judge Fernando Rodriguez said Donald Trump's use of the wartime power was "unlawful" and had been improperly invoked.

The law was written in 1798 to allow the removal of non-citizens in times of war or invasion. It has been used by Trump to deport Venezuelans to El Salvador on the basis they were members of the Tren de Aragua gang and "conducting irregular warfare".

The judge said the act only applied when the US was facing an "armed organised attack", which it is not. The administration has not commented.

President Trump in March issued a proclamation claiming that Tren de Aragua was invading the US, and justified the use of special powers to deport immigrants identified as gang members without court proceedings.

He used the act to deport two plane loads of alleged migrant gang members to the El Salvadoran terrorism prison known as Cecot.

Neither the US government nor El Salvador has provided details of the deportees' alleged criminality or gang membership.

The ruling is the first time a federal judge has ruled the use of the act is "unlawful".

"The historical record renders clear that the president's invocation of the AEA... is contrary to the plain, ordinary meaning of the statute's terms," the judge wrote.

"As a result, the court concludes that as a matter of law, the executive branch cannot rely on the AEA... to detain the named petitioners... or to remove them from the country."

The judge pushed back against the Trump administration's argument that the president's use of the act was beyond the purview of the courts.

"Allowing the president to unilaterally define the conditions when he may invoke the AEA, and then summarily declare that those conditions exist, would remove all limitations to the executive branch's authority under the AEA," he said

He added that allowing such a move would "strip the courts of their traditional role of interpreting Congressional statutes".

The Alien Enemies Act has only been used three times before in US history - during World War Two, World War One and the War of 1812.