The Pentagon said in a statement that 2,800 military units are located in Texas, 1,500 in Arizona and 1,300 in California.

Some 2,000 members of the National Guard, mobilized since last April, are also in the border area.

U.S. President Donald Trump threatened on October 31 to send up to 15,000 soldiers to the dividing line after the department announced it had sent 7,000 to that end.

The military publication Star and Stripes reported then that the Northern Command had confirmed that more soldiers could be added to the 5,239 initially assigned.

Nearly 40 units were summoned for the movement and the list includes combat engineers, Third Infantry Division aviators, members of forts from Texas, North Carolina and Kansas, and medical and logistical staff, the source added.

This operation, called Faithful Patriot, a name recently dropped, was described by Northern Command chief Terrence O'Shaughnessy as an effort to help the Bureau of Customs and Border Protection tighten defenses at legal entry points and nearby sites.

Newspaper reports stated that the military will have its actions limited by a federal law.

Such a rule, they pointed out, prevents the Army from carrying out police work in U.S. territory, which means that uniformed soldiers won't be able to participate directly in operations against Central American caravans seeking to reach the United States.

Some of the tasks allowed to soldiers include providing air support for border missions, installing concrete barriers, and repairing and maintaining vehicles.

R. Gil Kerlikowske, commissioner of the Bureau of Customs and Border Protection from March 2014 to January 2017, said the Army cannot stop asylum seekers at border crossings requesting protection.

I see the deployment of troops as a political maneuver and a waste of both military resources and taxpayers' money,' he said.

It is a big mistake to use active military personnel and assign them to such duties, Kerlikowske added.