Members of Congress requested an investigation three months ago into whether the deployment breaks regulations banning active duty military troops from carrying out domestic law enforcement duties.

The Department of Defense's independent inspector general sent a letter to Secretary of Defense Mark Esper and other high-level Pentagon officials, asking for their cooperation with the inquiry.

A sharp increase in people crossing into the US from Mexico, mostly Central Americans fleeing violence and poverty, has created a crisis, with detainees living in unsanitary conditions in overcrowded holding centers.

Some 3,900 active-duty troops and 2,600 National Guard reservists are deployed at the US-Mexico border, according to the Pentagon, with Esper having extended the mission through September 2020.

US soldiers do not have the right to search, detain or arrest civilians, in accordance with the Posse Comitatus Act, a 19th century law meant to prevent the military being used against US citizens.

The inspector general will look into their training on interacting with civilians and coordinating with other government agencies, and at their funding, according to the letter.

Troops were first deployed to the border last year. The number is expected to drop to 2,400 active duty soldiers by the end of the year.