WASHINGTON - A US appeals court on Wednesday upheld a lower court's temporary block on the Trump administration's deportation of some Venezuelan immigrants under a little-used 18th century law.

The decision by the US Circuit Court of Appeals for the DC Circuit marks a defeat for Republican President Donald Trump, who argued US District Judge James Boasberg's two-week ban on deportations under the 1798 Alien Enemies Act encroached on the executive's authority to make national security decisions.

A three-judge panel voted 2-1 to uphold Boasberg's block on deportations, with US Circuit Judge Justin Walker - who was appointed by Trump during his first term - dissenting.

Trump invoked the Alien Enemies Act on March 15 to swiftly deport alleged members of the Venezuelan gang Tren de Aragua, attempting to speed up removals with a law best known for its use to intern Japanese, Italian and German immigrants during World War Two.

An ensuing legal battle over the move has highlighted Trump's attempts to strong-arm the federal judiciary, a coequal branch of government that serves as a check on executive power.

Boasberg temporarily blocked the Alien Enemies Act deportations later on March 15 following a legal challenge by the American Civil Liberties Union.

But the Trump administration allowed two planes already in the air to continue to El Salvador where the US handed 238 Venezuelan men over to Salvadoran authorities to be placed in the country's "Terrorism Confinement Center."

The ruling came as US Homeland Security Secretary Kristi Noem visited El Salvador's mega-prison holding the Venezuelans deported by the US.

During a tour of the prison, Noem approached cells holding dozens of shirtless men clad in white pants, some with tattoos, who stared back silently. She visited one barracks where some Venezuelan detainees were being held, according to a pool report. The temperature in the room was "uncomfortably warm" - upwards of 32 degrees Celsius, the report said.

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When Noem left that room, there was "a sudden eruption of noise" that turned into a chant, but the words were unclear, the report said. The prisoners spend nearly the entire day in their cells, leaving for brief exercise or visits to a medical clinic, it said.

Noem, an outspoken proponent of Trump's immigration crackdown, also planned to meet with El Salvador's President Nayib Bukele, whose Trump-aligned government is holding the migrants in exchange for $6 million.

The US and El Salvador signed a memorandum of cooperation to update an existing security agreement between the two countries, Noem said on social media. She added the deal will ensure "fugitives' criminal records are shared between America and El Salvador."

'Nazis got better treatment'

Family members of many of the deported Venezuelan migrants deny the alleged gang ties. Lawyers for one of the deportees, a Venezuelan professional soccer player and youth coach, said US officials had wrongly labeled him a gang member based on a tattoo of a crown meant to honor his favorite team, Real Madrid.

In explaining her decision to uphold Boasberg's ruling, US Circuit Judge Patricia Millett, an appointee of Democratic President Barack Obama, said the government was not affording the migrants the chance to contest the government's assertion that they were members of Tren de Aragua before deporting them.

"Nazis got better treatment under the Alien Enemies Act than has happened here," Millett said at a hearing on Monday over the Trump administration's request the appeals court halt Boasberg's order.

Drew Ensign, a lawyer for Trump, responded, "We certainly dispute the Nazi analogy."

US Circuit Judge Karen Henderson, an appointee of Republican President George H.W. Bush, said it was not clear that Tren de Aragua's presence in the United States constituted an act of war as contemplated by the Alien Enemies Act.

"An invasion is a military affair, not one of migration," Henderson wrote in her ruling on Wednesday.

Ensign argued on Monday that the judge had no right to second-guess the president's decision on foreign affairs matters.

In a dissenting opinion, Walker wrote, "The government likely faces irreparable harm to ongoing, highly sensitive international diplomacy and national-security operations."

Walker also said he would have halted Boasberg's ban because the proper forum for the migrants to bring the case was in Texas, where they are detained, rather than in Washington, DC.

The Trump administration may ask the US Supreme Court, which has a 6-3 conservative majority, to review the matter.

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Lee Gelernt, the lead ACLU attorney in the case, said the appeals court ruling "stops the immediate removal of hundreds of individuals, without any due process, to a notorious prison".

Boasberg is separately weighing whether the Trump administration violated his order by failing to return deportation flights after his order was issued.