The US Supreme Court on Saturday paused the Trump administration's deportation of alleged Venezuelan gang members under an 18th-century law.

US President Donald Trump invoked the 1798 Alien Enemies Act (AEA) last month to begin rounding up Venezuelan migrants accused of belonging to the Tren de Aragua gang before expelling them to a maximum security prison in El Salvador.

The obscure law has only previously been used during the War of 1812, World War I and World War II.

"The government is directed not to remove any member of the putative class of detainees from the United States until further order of this court," the Supreme Court's brief order issued early Saturday said.

The order came after rights lawyers filed an emergency appeal to halt the deportation of migrants currently held in a facility in the southern state of Texas.

The American Civil Liberties Union (ACLU) said in its emergency filing on Friday night that the group of Venezuelans held in Texas had been told "they will be imminently removed under the AEA, as soon as tonight." 

Attorneys for several of the Venezuelans previously deported had said their clients were not members of Tren de Aragua, had committed no crimes and were targeted largely on the basis of their tattoos.

Trump, who campaigned on a pledge to expel millions of undocumented migrants, has accused Venezuela of "perpetrating an invasion" of the United States through the entry of alleged Tren de Aragua members.

The Supreme Court said this month that anyone facing deportation under the AEA must first be given an opportunity to legally challenge their removal.

'Imminent removal'

The ACLU said in its filing on Friday that the migrants in Texas were in danger of "being removed from the United States without notice or an opportunity to be heard."

"Many individuals have already been loaded on to buses, presumably headed to the airport," the rights group said.

The Trump administration went ahead with the initial deportations of alleged Tren de Aragua members under the AEA in March despite an order by federal judge James Boasberg blocking the move.

Judges and lawmakers are now wrangling with Salvadoran officials over the fate of Kilmar Abrego Garcia, a Maryland resident who was deported last month due to what the White House later said was an "administrative error."

The Supreme Court lifted the block on 7 April, in the same decision where it said people facing deportation are entitled to due process.

The deported migrants are currently held in El Salvador's maximum security Terrorism Confinement Center, a mega-prison southeast of the capital San Salvador with capacity for 40,000 prisoners.

Inmates there are packed in windowless cells, sleep on metal beds with no mattresses and are forbidden visitors.