The administration of US President Donald Trump says it has deported hundreds of alleged gang members by invoking the wartime "Alien Enemies Act" despite a court order to bar the move.

On Saturday, President Trump signed a proclamation to invoke the act, targeting what he referred to as Venezuelan gang members in the US. The White House calls them a direct threat to national security.

The Alien Enemies Act allows the president to detain or deport the citizens of an enemy nation in a time of war without undergoing normal court procedures. It was used to detain immigrants from Japan and elsewhere during World War Two.

Also on Saturday, a federal court in Washington issued an order temporarily blocking the deportations under the act. The court also ordered any airplanes carrying deported migrants to return to the US immediately.

The court cited the possibility that conditions required to apply the law may not have been fulfilled in this case.

US Secretary of State Marco Rubio said in a statement on Saturday that the US sent hundreds of members of a Venezuelan gang to El Salvador which has agreed to house them in jails.

Salvadoran President Nayib Bukele wrote in a social media post, "Today, the first 238 members of the Venezuelan criminal organization, Tren de Aragua, arrived in our country."

He described the US court order to stop the deportation flights as "too late."

The invocation of the Alien Enemies Act has caused controversy in the US and friction between the courts and the Trump administration.