Washington DC - A federal judge ruled on Wednesday that the Trump administration policy of deporting undocumented migrants to countries that are not their own is unlawful.

District Judge Brian Murphy put his ruling on so-called "third-country deportations" on hold for 15 days to allow the government to file an appeal.

"This case is about whether the Government may, without notice, deport a person to the wrong country, or a country where he is likely to be persecuted, or tortured," Murphy wrote.

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The Department of Homeland Security has said it is "fine" to deport a migrant to a third country so long as DHS knows they will not be shot on arrival, Murphy said.

"It is not fine, nor is it legal," he said.

The judge noted that Congress has made it US policy not to deport people to countries where their lives would be in danger or they could be subject to torture.

Under the DHS policy, however, "immigration officers need not give notice or any opportunity to object before removing someone to an unfamiliar and potentially dangerous country," Murphy said.

A person may be deported if the government has received "'assurances' that no persecution or torture will happen there," he said.

"This new policy – which purports to stand in for the protections Congress has mandated – fails to satisfy due process for a raft of reasons, not least of which is that nobody really knows anything about these purported 'assurances,'" Murphy said.

"Whom do they cover? What do they cover? Why has the Government deemed them credible? How can anyone even know for certain that they exist?" he asked.

Murphy, an appointee of Democratic President Joe Biden, previously sought to block the deportation of a group of migrants to war-torn South Sudan but was overruled by the conservative-dominated Supreme Court.

According to the US authorities, the eight men – two from Myanmar, two from Cuba, and one each from Vietnam, Laos, Mexico, and South Sudan – were convicted violent criminals.

The Trump administration has claimed third-country deportations are necessary because the home nations of some of those who are targeted for removal sometimes refuse to accept them.