(Reuters) - The American Civil Liberties Union of New Mexico on Thursday called for state authorities to investigate a small group of armed U.S. citizens who they alleged are illegally detaining migrants entering the United States.

FILE PHOTO: A child looks through the border wall during the visit of U.S. President Donald Trump to Calexico, California, as seen in Mexicali, Mexico April 5, 2019. REUTERS/Carlos Jasso

The United Constitutional Patriots, who claim to be mainly military veterans, have been patrolling the U.S.-Mexico border near Sunland Park, New Mexico, since late February in search of illegal border crossers.

They post near daily videos showing members dressed in camouflage and armed with semi-automatic rifles holding groups of migrants, many of them Central American families seeking asylum, until U.S. Border Patrol agents arrive to arrest them.

The small volunteer group says it is helping Border Patrol deal with a surge in undocumented migrants but civil rights organizations like the ACLU say it is a “fascist militia organization” operating outside the law.

“We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum,” the ACLU said in a letter to New Mexico Governor Michelle Lujan Grisham and Attorney General Hector Balderas.

“We urge you to immediately investigate this atrocious and unlawful conduct.”

The offices of Lujan Grisham and Balderas did not respond to requests for comment.

On a March 27 visit to El Paso, Texas, next to Sunland Park, then U.S. Customs and Border Protection (CBP) Commissioner Kevin McAleenan said his agency, which runs U.S. Border Patrol, did not need the help of citizens to police the border.

“We are not asking for civil society groups to provide border security assistance,” said McAleenan, who was recently appointed acting secretary of the U.S. Department of Homeland Security.

U.S. CBP did not respond to a request for further comment. UCP member John Horton did not immediately return calls. Horton has previously told media that UCP members are armed for self defense, as is their right under U.S. law, and aware they cannot detain people entering the United States illegally.

U.S. armed groups have long patrolled the U.S. border, their numbers rising during upticks in migrant apprehensions, such as during the mid 2000s when the Minuteman Project was established.

The UCP says it is responding to a rise in migrant arrests to their highest monthly levels in more than a decade.

The ACLU said the group was a product of the Trump administration’s “vile racism” that “has emboldened white nationalists and fascists to flagrantly violate the law.”

(This story adds dropped words in paragraph 8)