Washington, Apr 21 (AFP) The FBI has arrested a member of an armed rightwing militia group accused of illegally detaining migrants at the US-Mexico border, officials said Saturday.

Larry Mitchell Hopkins, 70, was arrested for illegal possession of a weapon. His small but well-armed United Constitutional Patriots (UCP) group says it watches the border, stands guard over people who want to hand themselves in to border patrol and pursues those who don't.

New Mexico Attorney General Hector Balderas described Hopkins, who also goes by the name of "Striker," as a "dangerous felon who should not have weapons around children and families."

"Today's arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not armed vigilantes," he added.

Hopkins was said to have been detaining migrants near Sunland Park, New Mexico.

In recent months, thousands of migrants have arrived in Mexico, primarily Central Americans fleeing poverty and violence at home.

US President Donald Trump has described them as a threat to national security, demanding billions of dollars from Congress to build a wall on the southern border.

UCP members insist they will patrol the border until the wall is built.

"We're here to assist the border patrol because they are so short handed," Hopkins told AFP earlier this month.

The group -- made up mostly of older veterans -- enjoys an enthusiastic social media following. They are equipped with tactical gear, rifles and even custom patches.

Hopkins could be heard during lengthy radio broadcasts streamed online several times a week.

The border vigilante movement is well-established, with the Southern Poverty Law Center documenting "questionable apprehensions of migrants by private citizens" from as far back as 1999.

In a statement Saturday, the Mexican foreign ministry expressed "deep concern" about such groups.

"This type of practice can lead to a violation of the human rights of people who migrate or request asylum or refuge in the United States," it said. (AFP)

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