The U.S. government is tracking three separate migrant caravans, one of them estimated to be more than 12,000 people in size, heading north toward the U.S., a top Pentagon official said Tuesday.

Defense Undersecretary for Policy John Rood told Congress that those caravans are one reason why the presence of active-duty troops is still needed on the U.S.-Mexico border.

He also said military medical personnel are helping with the latest border problem, dealing with an increase in sick migrants showing up at the border and in need of immediate attention.

Some 2,300 active-duty troops are still on the border, down from 5,900 who were initially ordered deployed in late October, ahead of previous caravans. That’s in addition to about 2,000 National Guard troops who have been on the border since earlier last year.

Mr. Rood said some of the active-duty troops will be on the border through Sept. 30.

New House Armed Services Committee Chairman Adam Smith questioned why the mission was expanded from guard to active-duty troops, calling that a “very rare” move.

Vice Admiral Michael Gilday, director of operations at the Joint Chiefs of Staff, said they just didn’t have enough capacity within the National Guard to fulfill the requests President Trump and the Homeland Security Department made.

“We didn’t have those skill sets available in the guard to draw on at the time,” he said.

The hearing is the first by the new Democratic majority in the House taking a look at the Trump administration’s border policies.

Democratic lawmakers questioned the tradeoffs of deploying troops to the border, giving the other competing needs on the military.

“We’ve got other needs in the world,” Mr. Smith said. “This is not primary to our mission and if we start down this road … and say DOD is all about the border, where does it stop?”

Admiral Gilday said the one area where the military is “a bit pinched” in providing support is in helicopters, which are highly sought in war theaters overseas.

He said, though, that he doesn’t consider the deployment only a drain, but a chance to train and build readiness.

The troops were called late last year to harden the official border crossings, laying 70 miles of razor wire near the ports of entry, which Homeland Security says has been critical in preventing caravans from successfully breaking through.

The defense officials said moving forward, they expect the troops will be used more in between the official crossings.

“We bring a skill set with respect to detection and monitoring that I think is going to be very valuable for CBP,” Admiral Gilday said.

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