US Homeland Security Secretary Kristi Noem has visited a controversial mega-prison in El Salvador, to which US officials sent 238 Venezuelans.

The Trump Administration alleges that the men are members of Venezuelan gang Tren de Aragua and had them deported to El Salvador under the 1798 Alien Enemies Act.

The move has prompted an outcry in the US and Venezuela, with several family members of the deportees insisting their relatives do not belong to any gang.

Noem's visit to the country, and to the jail known as Terrorism Confinement Centre (Cecot) in particular, is seen as a show of support for Salvadorean President Nayib Bukele, as pressure against housing deportees at the prison grows.

As part of her visit, Noem is due to meet Bukele, an ally of US President Donald Trump, who had the mega-jail built as part of his own crackdown on gang crime in El Salvador.

President Bukele made the offer to incarcerate deportees and prisoners from the US at the Cecot during a recent visit to the Central American nation by Secretary of State Marco Rubio.

As Secretary Noem was being shown around the notorious facility on Wednesday, the Trump Administration received another setback in its effort to send foreign nationals there.

An appeals court in Washington DC upheld a decision by a lower court to put a temporary injunction on the deportation of Venezuelan immigrants under the 1798 act, which allows for the expulsion of foreign citizens with little due process.

The use of the act has drawn an outcry from immigration lawyers and activists who argue that some men accused of being gang members have been sent to El Salvador and locked up in the mega-prison on the basis of little evidence, including simply having tattoos.

Human rights groups have warned that the jail, in which inmates are held in windowless cells and sleep on bare metal bunks, is a "concrete and steel pit".

The White House continues to insist that all those they rounded up are dangerous gang members and were carefully vetted.

However, several of their family members in Venezuela say their loved ones had no prior convictions.

The deportation of 238 Venezuelans to the Cecot earlier this month has also put the Trump Administration into open conflict with a federal judge, James Boasberg, over the use of the centuries-old law to justify their quick deportation.

Judge Boasberg has since imposed an injunction on further deportations under the law, which was last invoked during World War Two.

The fate of the Venezuelans has also come under severe criticism by a Washington Court of Appeals judge who said that even "Nazis got better treatment under the Alien Enemies Act".

The comparison prompted a furious reaction from senior officials in the Trump Administration.

Meanwhile, lawyers in El Salvador, apparently acting in coordination with the Venezuelan government, have lodged a petition with the Salvadorean Supreme Court to try to secure the immediate release of the men.

The deportations to El Salvador under Trump's second term are part of the president's long-running campaign against illegal immigration in the US.

He won over voters on the campaign trail, in part, by promising to enact the largest deportation operation in US history.

In January, Trump signed an executive order declaring Tren de Aragua and MS-13 foreign terrorist organisations.

While irregular border crossings have plummeted to the lowest number in decades since Trump took office, the Republican president has reportedly been frustrated by the relatively slow pace of deportations so far.