A militia group near the US-Mexico border detained hundreds of people this week, New Mexico's attorney general told CNN.

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

Videos posted online purportedly showing migrants held by the United Constitutional Patriots group and handed over to the US Border Patrol drew swift condemnation from the American Civil Liberties Union of New Mexico.

"We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum," the ACLU said in a letter to state authorities denouncing the actions and asking the government to step in. "We urge you to immediately investigate this atrocious and unlawful conduct."

CNN has reached out to the United Constitutional Patriots for comment.

The New York Times reported that a spokesman for the militia group said their actions were legal, "comparing the detention of the migrants to 'a verbal citizen's arrest.'"

Videos show armed men in masks, fatigues

Various private militia groups -- often espousing anti-immigrant views -- have patrolled the border for years.

But it's rare to see video of an armed group detaining migrants on the US side of the border.

Authorities say they're looking into videos posted on the United Constitutional Patriots New Mexico Border Ops Facebook page.

CNN has reached out to the group and the individuals who posted the videos for comment but has not confirmed details about who shot the videos, or when and where they were recorded.

The videos purport to show members of the group detaining migrants, including families with children, who've just crossed the border.

They show people often in full military fatigues, with handguns strapped to their sides, wearing gloves and black face masks. Armed men order migrants to stop, force them to sit on the ground and then apparently call Border Patrol to pick them up. At least two videos posted on the group's Facebook page depict a man in fatigues verbally identifying himself as "Border Patrol" as he stops a group of migrants. (CNN)

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