Defense Secretary Jim Mattis says he will be heading to the U.S.-Mexico border on Wednesday, where troops are helping erect barriers and performing other tasks in support of border security.
About 5,800 active duty troops are assigned to the border mission. Of those, about 1,000 are on or near the border in south Texas. President Donald Trump ordered troops to the border in response to a caravan of migrants slowly making its way through Mexico toward the U.S.
The U.S. government said it was starting work on Tuesday to "harden" the border crossing from Tijuana, Mexico, to prepare for the arrival of a diminished migrant caravan leapfrogging its way across western Mexico.
In earlier comments, Mattis said the military's mission on the border has not changed "at this time," even though the lead migrant caravan is no longer headed toward south Texas. The caravan is now in western Mexico, with most of the migrants appearing to be headed toward Tijuana.
Customs and Border Protection announced it was closing four lanes at the busy San Ysidro and Otay Mesa ports of entry in San Diego, California.
It said the closures were needed "to install and pre-position port hardening infrastructure equipment in preparation for the migrant caravan and the potential safety and security risk that it could cause."
The caravans became a campaign issue in U.S. midterm elections and Trump ordered the deployment of over 5,000 military troops to the border to help fend off the migrants.
The migrants have come about 1,500 miles (2,400 kilometers) since they started out in Honduras around Oct. 13.
While they previously suffered from the heat on their journey through Honduras, Guatemala and southern Mexico, they now trek along highways wrapped in blankets to fend off the morning chill.
While the caravan previously averaged only about 30 miles (50 kilometers) a day, the migrants are now covering daily distances of 185 miles (300 kilometers) or more, partly because they are relying on hitchhiking rather than walking.