(CNN) - The approximately 1,300 US troops on the Texas southern border are expected to finish their assigned task of reinforcing border crossing points, largely with concertina wire, in the coming days. Once that work is done, it's not clear what additional orders they will be given other than putting up more wire, according to two defense officials.
As of Tuesday there are 5,900 US troops assigned to the border mission in Texas, Arizona and California, according to one of the defense officials. Several hundred troops, however, remain at their various headquarters as planners, while others are part of aviation units.
While there are approximately 2,800 troops assigned to the mission of helping protect the Texas frontier, only 1,300 are on the border.
Border reinforcements at McAllen have largely been completed, the official said. Similar reinforcements at Brownsville were expected to be completed Tuesday.
Reinforcements are also expected to be carried out at Laredo crossing points. However, it is not clear how many of the troops will stay on the border and how they will assist Customs and Border Protection officers when the Central American migrants who spurred the mission reach the border. There have been separate discussions about troops assisting in the movement of border control officers by air.
Secretary of Defense James Mattis said Tuesday that "there's been no change" to the mission by the US military.
"Right now, the mission is exactly what it is. Basically, we're doing the obstacle emplacement, we do have helicopters down there to move the commissioner of Border Patrol's people around, but there's been no change to that mission at this time. We'll have to see what the future holds but right now that's the only mission I have," he said.
There is no indication whether troops will be sent back to their home bases before the scheduled December 15 end of the mission.
On Tuesday Mattis announced he will head to the southern border on Wednesday to visit the troops who have been deployed.
The one-day trip will include a stop in Texas, where Mattis will meet with service members, the Pentagon announced.
Secretary of Homeland Security Kirstjen Nielsen will also visit the border in Texas on Wednesday, where she will do a tour with the commander of Northern Command, Gen. Terrence O'Shaughnessy, according to a Department of Homeland Security official speaking on condition of anonymity.
The visits by Mattis and Nielsen to Texas comes as a number of large groups of migrants are making their way through Mexico. Several dozen migrants from the so-called caravan that drew President Donald Trump's ire before the midterm elections have made it to Tijuana, Mexico.
A much larger group of thousands of migrants is still more than 1,000 miles away from that border city, according to Pueblo Sin Fronteras, an organization that's been working with the migrants. And depending how the group travels, it could take them days or weeks more to reach the border.
The current US government assessment is that about 100 migrants are at the Tijuana border crossing. About 1,100 Marines in California are assigned to provide support for border authorities there and are beginning to work on reinforcing cross points.
Other migrant groups include about 3,500 persons near the Mexican city of Guadalajara and 1,500 near Veracruz, according to the assessment.
Currently there is no indication that a migrant caravan is headed toward the Texas area, which includes the crossings at Brownsville and McAllen, although that could quickly change, according to a defense official who noted the migrant groups are still inside Mexico and could decide to move toward a Texas crossing.
But for now, that has not happened, the official said.
Asked Tuesday whether the orders could change for the military if migrants continue to head toward Tijuana and not the Texas border, Mattis said the mission won't change.
"No. We meet daily on this issue and coordinate amongst us. The first thing that's briefed is what is the threat against the border, the people who are trying to violate the border, but for right now there's no change in that mission set," he said.
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