Roughly half of the 5,900 active duty troops sent to the U.S.-Mexico border have returned home, a Pentagon spokesman said Monday.

Army Col. Rob Manning told reporters at the Pentagon that 3,150 active-duty troops remain at the southern border, including 1,200 in California, 1,050 in Texas and 900 in Arizona.

“The department will continue to assess force requirements to meet CBP, Customs and Border Protection, needs,” Manning said. "Whatever support the CBP and [Department of Homeland Security] needs is what we're going to provide them."

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Manning said last week that 5,200 service members were at that border, down from its high of about 5,900 in November. The newest number represents a drop of 2,050 military personnel.

The mission — ordered by President Trump shortly before the midterm elections to respond to what he called an "invasion" of thousands of Central American immigrants seeking asylum — was originally meant to last through Dec. 15.

Homeland Security then requested an extension of the mission through Jan. 31, which Defense Secretary James Mattis signed off on earlier this month.

Manning would not say how many service members will remain at the border through the end of the mission, adding, “be very cautious about timelines.”