A US judge has ruled that the Trump administration must return a Maryland man who was wrongly deported to El Salvador to the United States within three days, the latest legal setback for the administration's hardline deportation policies.

The US had already acknowledged Kilmar Abrego Garcia – a Salvadoran migrant who lived in the US legally with a work permit – was deported in error last month when the government flew three planes of migrants alleged to have ties to violent gangs or immigration violations to the Central America country.

But the administration has argued it has no legal authority to bring him back to the country, though Abrego Garcia's lawyer Andrew Rossman, of prominent law firm Quinn Emanuel who joined Abrego Garcia’s legal team on Friday, disputed that.

"They put him there, they can bring him back,” Rossman said.

After questioning government lawyers, US District Judge Paula Xinis ruled at a Greenbelt, Maryland, court hearing that the government must take steps to bring him back to the United States by Monday.

The Justice Department will appeal the order to the Richmond-based 4th US Circuit Court of Appeals, according to a court filing after the hearing.

White House Press Secretary Karoline Leavitt said Xinis should contact President Nayib Bukele of El Salvador "because we are unaware of the judge having jurisdiction or authority over the country of El Salvador".

In a court filing, the US Justice Department called the judge's order "indefensible" and urged the appeals court to immediately pause the ruling.

The United States said Abrego Garcia "has no legal right or basis to be in the United States at all" and that "the public interest obviously disfavors his return, let alone a slapdash one conducted as the result of judicial fiat".

Earlier, at the hearing, Abrego Garcia's lawyer Simon Sandoval-Moshenberg told the judge that there was no legal basis for the deportation.

“They admit they had no legal authorisation to remove him to El Salvador,” Moshenberg said.

“The public interest lies in the government following the law.”

Erez Reuveni, a lawyer for the government, conceded that Abrego Garcia should not have been removed.

"That is not in dispute," Reuveni said.

In an unusual exchange, Xinis grilled Reuveni on why the US couldn't get Abrego Garcia back – to which Reuveni said he had asked US government officials that question without getting a satisfactory answer himself.

"The absence of evidence speaks for itself,” Reuveni said.

Reuveni, a senior Justice Department immigration lawyer, was later put on indefinite leave from his job, according to the New York Times, citing documents it reviewed. The Justice Department did not immediately respond to a request for comment.

Attorney General Pam Bondi told the newspaper in a statement: “At my direction, every Department of Justice attorney is required to zealously advocate on behalf of the United States.

"Any attorney who fails to abide by this direction will face consequences.”