Washington DC - A federal judge in Texas ruled on Thursday that President Donald Trump's use of an obscure wartime law to summarily deport alleged Venezuelan gang members was "unlawful."

District Judge Fernando Rodriguez, a Trump appointee, blocked any deportations from his southern Texas district of alleged members of the Tren de Aragua (TdA) gang using the 1798 Alien Enemies Act (AEA).

Trump invoked the little-known AEA, which was last used to round up Japanese-American citizens during World War II, on March 15 and flew two planeloads of alleged TdA members to El Salvador's notorious maximum security CECOT prison.

In a proclamation, Trump said Tren de Aragua was engaged in "hostile actions" and "threatening an invasion or predatory incursion against the territory of the United States," adding that Venezuela's Nicolas Maduro was pulling the strings.

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The Supreme Court and several district courts have temporarily halted removals under the AEA, citing a lack of due process, but Rodriguez is the first federal judge to find that its use is unlawful.

"The president cannot summarily declare that a foreign nation or government has threatened or perpetrated an invasion or predatory incursion of the United States," the judge said in his 36-page order.

"Allowing the president to unilaterally define the conditions when he may invoke the AEA, and then summarily declare that those conditions exist, would remove all limitations to the executive branch's authority under the AEA," Rodriguez said.

"The president's invocation of the AEA... exceeds the scope of the statute and, as a result, is unlawful," the judge said.

The administration does not have the lawful authority, under the AEA, "to detain Venezuelan aliens, transfer them within the United States, or remove them from the country," he added.

Lee Gelernt, an attorney with the American Civil Liberties Union (ACLU), which brought the lawsuit seeking to halt the removals, welcomed the ruling.

"The court ruled the president can't unilaterally declare an invasion of the United States and invoke a wartime authority during peacetime," Gelernt said.

"Congress never meant for this 18th-century wartime law to be used this way," he said. "This is a critically important decision that prevents more people from being sent to the notorious CECOT prison."

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While prohibiting deportations under the AEA, Rodriguez said the administration could proceed with removals under the Immigration and Nationality Act.

Trump won the White House election last November in large part on promises to combat what he repeatedly claimed is an invasion of criminal migrants.

Trump has sent troops to the Mexican border, imposed tariffs on Mexico and Canada for allegedly not doing enough to stop illegal crossings, and designated gangs like TdA and MS-13 as terrorist groups.

Attorneys for several of the Venezuelans already deported have said their clients were targeted largely on the basis of their tattoos.

In the most publicized case to date, Maryland resident Kilmar Abrego Garcia was deported to CECOT before the Trump administration admitted he was sent there due to an "administrative error."