Washington DC - A federal judge took aim Thursday at the Trump administration's "shocking" position that it can do nothing to return a migrant wrongfully deported to El Salvador, slamming it as an affront to liberty.

"The government is asserting a right to stash away residents of this country in foreign prisons without the semblance of due process that is the foundation of our constitutional order," Judge Harvie Wilkinson wrote on behalf of the unanimous three-member appellate panel, which denied the government's efforts to halt the case.

"This should be shocking not only to judges, but to the intuitive sense of liberty that Americans far removed from courthouses still hold dear," wrote the judge, an appointee of Republican president Ronald Reagan.

The Trump administration has admitted that Kilmar Abrego Garcia, who was living in Maryland and married to a US citizen, was deported to a notorious prison in El Salvador due to an "administrative error."

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Another judge has ordered President Donald Trump to "facilitate" his return – an order upheld by the Supreme Court – but the US government has said the court did not have the authority to order it to have him returned.

Trump has alleged that Abrego Garcia is "an MS-13 Gang Member and Foreign Terrorist from El Salvador," while Press Secretary Karoline Leavitt claimed that he was "engaged in human trafficking."

He has never been charged with any crimes.