A group of armed vigilantes has been detaining large groups of migrants at the US-Mexico border at gunpoint while they await Border Patrol, videos show.

New Mexico officials, as well as Customs and Border Protection, have urged such militias not to take the law into their own hands.

But a spokesman for the group, Jim Benvie, told The New York Times the group's actions were legal, saying it was akin to "a verbal citizen's arrest."

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Armed, right-wing vigilantes have been detaining large groups of migrants crossing the US-Mexico border and holding them at gunpoint while they wait for Border Patrol agents to arrive, videos posted to YouTube show.

One such video shows a group of nearly 300 migrants sitting on the ground near Sunland Park, New Mexico, as members of the United Constitutional Patriots shines flashlights in their faces.

A spokesman for the group, Jim Benvie, told The New York Times the group's actions were legal, saying it was akin to "a verbal citizen's arrest." He added that the group will stay at the border until President Donald Trump's wall is built, or until America's asylum laws are changed.

"We're just here to support the Border Patrol and show the public the reality of the border," he said.

The news comes amid a surge of Central American migrant families seeking asylum in the US and infuriating the Trump administration. More than 100,000 migrants crossed the border last month, and that number is expected to rise in April.

Rights groups like the American Civil Liberties Union have denounced the group, urging New Mexico's governor and attorney general in a letter to immediately investigate the group's "atrocious and unlawful conduct."

"The vigilante members of the organization … are not police or law enforcement and they have no authority under New Mexico or federal law to detain or arrest migrants in the United States," two ACLU lawyers said in the letter. "The Trump administration's vile racism has emboldened white nationalists and fascists to flagrantly violate the law."

New Mexico's attorney general, Hector Balderas, denounced the group's actions in a brief statement.

"My office has been informed that this week, an armed group has detained nearly 300 people near Sunland Park, New Mexico," Balderas said. "These individuals should not attempt to exercise authority reserved for law enforcement."

Customs and Border Protection also said in a statement Friday it doesn't endorse or condone private groups taking "matters into their own hands," and that community members who witness or suspect "illegal cross-border activity" can call Border Patrol's tip line.

"Interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved," the agency tweeted.