The first groups of caravan migrants have reached the US-Mexico border and are staying in shelters in Tijuana, Mexico.
Thousands more lag far behind the initial group, and Tijuana officials expect them to arrive within weeks.
The US government has responded by shutting down lanes at the San Diego ports of entry and setting up barriers and wire.
The roughly 5,200 troops that the Trump administration deployed ahead of the caravan are still in Texas — hundreds of miles away from where the caravan is arriving.
The first groups of caravan migrants reached the United States' southern border this week, hundreds of miles ahead of the main caravan of thousands of other migrants traveling north through Mexico.
Roughly 800 migrants have reached Tijuana, The New York Times reported on Wednesday.
The US Customs and Border Protection agency has responded by attempting to "harden" San Diego's border with Tijuana, shutting down four lanes at the San Ysidro and Otay Mesa ports of entry. The agency also said in a statement that US troops are putting up concertina wire, barriers, and fencing.
Officials in Tijuana have arranged shelter beds for the migrants already there, but city officials have expressed some alarm over the rapidly growing number of migrants arriving.
See more related to this story:
The director if the Tijuana Municipal Migration Affairs Office told The Washington Post there is only shelter space for 900 migrants, though they expect at least 3,000 more to arrive in the next couple of weeks.
"We are expecting more of these little groups, but we don't know right now exactly what we are going to do with the big caravan," César Palencia told The Post.
'To be determined'
The migrants are largely from Central American countries and say they plan to seek asylum in the United States, though the American ports of entry are letting in only a few migrants at a time to make claims.
It's still unclear when the main group of migrants will arrive, as many are traveling through the Mexican states of Jalisco and Nayarit. Though the smaller groups of migrants that have splintered off have managed to arrange bus transportation, the larger groups haven't moved as quickly.
Meanwhile, roughly 1,500 miles away, troops stationed at the US-Mexico border in south Texas met with top Trump administration officials on Wednesday to discuss their mission.
In anticipation of the caravan, roughly 5,200 troops were deployed to the area, where they have been erecting barriers and putting up wires — even though the caravan is arriving at the opposite end of the country.
During a live-streamed meeting with Defense Secretary Jim Mattis, one soldier asked about the short- and long-term plans for the mission. Mattis responded that the latter was "to be determined."
"When you're in something like this," he said, "It's dynamic, it's unpredictable. We'll have to see."