Increasingly sick and facing a punishing 60-mile (100 kilometer) trek, members of the migrant caravan began leaving the southern Mexico city of Pijijiapan on Friday and walking in the pre-dawn darkness to the next stop, Arriaga.

The migrants’ coughs could be heard in the darkness long before their faces became visible.

Central American migrants traveling with a caravan to the U.S., prepare to leave Mapastepec, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

Yamileth Caldames, one of those making the arduous journey, went to the highway, took a look at what lay ahead and returned to town with her sleeping 3-year-old daughter in her arms. Her 5-year-old daughter walked alongside her, while the children’s father pushed an empty stroller through the dark.

“My blood pressure is bad,” Caldames said.

With what little money they still had, they planned to buy bus tickets most of the way to Arriaga to try to regain their strength.

But if Mexican police catch them riding a bus, they could tell the driver to drop them off on the road. Authorities are enforcing an obscure highway insurance rule in an apparent bid to make families like the Caldemes walk as much of the way as possible, or dissuade them from continuing the journey at all.

A Mexican police car passes Central American migrants traveling with a caravan to the U.S. as they get a free ride on a tuck bed, as they approach Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

Many of the migrants already had blistered feet before they reached Pijijiapan on Thursday, and the town’s main plaza quickly became a makeshift triage center as the caravan of about 4,000 Central Americans arrived.

Central American migrants traveling with a caravan to the U.S. make their way to Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

A severely dehydrated woman connected to an IV line sat on a plastic chair in a gazebo. Nearby, volunteer nurses took temperatures and treated coughs, handing out donated medicine as migrants lined up.

Two weeks of walking have taken a toll on the migrants as they slowly make their way through Chiapas, Mexico’s southernmost state.

On Thursday, Dr. Jesus Miravete treated more than 120 people. Many had burns on their feet from walking in plastic sandals on the steaming highway.

“So many tell me: ‘I can’t rest. I have to go on,’” Miravete said. “It’s really hard. I feel overwhelmed, above all by the number of dehydrated children I have seen.”

Central American migrants rest as a thousands-strong caravan slowly making its way toward the U.S. border stops for the night in Pijijiapan, Chiapas state, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rebecca Blackwell)

As in many places in Chiapas, residents in Pijijiapan turned out in force to aid the travelers as they streamed in on foot, offering shelter, food and medical treatment. Some people offered rides to the plaza. Others showed up with used clothes and boxes of sandwiches.

The caravan was welcomed in a similar fashion into Mapastepec, a municipality of 45,000 residents 30 miles (48 kilometers) to the south where city officials put up tents around the main square offering everything from medical attention to donated clothing to baby formula. Local churches offered free showers and set up food distribution points.

Migrants receive donated food at a makeshift camp set up by a caravan of Central American migrants traveling to the U.S, in Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

“They are human beings. You have to do something to help them,” said Cesar Cabuqui, who handed out dozens of homemade bean and cheese sandwiches and bags of water.

Grateful for the hospitality, many of the migrants have tried to be respectful visitors.

Jose Reyneri Castellanos, from El Progreso, Honduras, hung back behind the rest of the caravan with his wife and two young sons to help sweep and tidy up in Mapastepec as he’d done at each stop, figuring it well help ensure a continued warm reception as they head north.

A small group of Honduran migrants trying to reach the U.S. border walk along train tracks in Trancas Viejas, Veracruz state, Wednesday, Oct. 24, 2018. (AP Photo/Rebecca Blackwell)

“I think it is important to leave the community and the city clean,” Castellanos said.

Many of the migrants say they are dreaming of finding better lives in the United States. They say they have been driven to leave their homelands by severe poverty and rising gang violence.

Such caravans have taken place regularly, if on a smaller scale, over the years, but U.S. President Donald Trump has seized on the phenomenon this year. At recent rallies and on Twitter, he has been talking about the caravan and illegal immigration, repeatedly hitting Democrats with the issue as the U.S. heads into the hotly contested Nov. 6 midterm elections.

Central American migrants traveling with a caravan to the U.S. get a free ride on a tuck bed from a driver, as they approach Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

Central American migrants traveling with a caravan to the U.S. in make their way to Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

Central American migrants traveling with a caravan to the U.S. make their way to Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

The Pentagon on Friday approved a request for additional troops at the southern border, likely to total several hundred, to help the U.S. Border Patrol as Trump seeks to transform fears about immigration and the caravan into electoral gains.

Defense Secretary Jim Mattis signed off on the request for help from the Department of Homeland Security and authorized the military staff to work out details such as the size, composition and estimated cost of the deployments, according to a U.S. official.

Central American migrants hitch a ride, as a thousands strong caravan heads north hoping to reach the U.S. border, near Mapastepec, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rebecca Blackwell)

Liseth Lopez, 23, from Comayagua, Honduras, rests on a sidewalk as she travels with a half dozen Honduran friends trying to reach the U.S. border, in Trancas Viejas, Veracruz state, Wednesday, Oct. 24, 2018. (AP Photo/Rebecca Blackwell)

Mattis, who is traveling in the Middle East, is expected to approve the actual deployments after all the details are ironed out, according to the official, who spoke on condition of anonymity to discuss planning that has not yet been completed or publicly announced.

The caravan is still some 1,000 miles (1,609 kilometers) from the nearest border crossing at McAllen, Texas, but the journey could be twice that if the migrants head to the Tijuana-San Diego crossing. That was the destination of a smaller caravan earlier this year, and only about 200 in the group made it.

A Honduran migrant carries his national flag as he walks north as part of a thousands strong caravan trying to reach the U.S., still over 1000 miles away, near Mapastepec, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rebecca Blackwell)

A Honduran couple, who received medical treatment for burned faces and his injured feet, rest together as a thousands-strong caravan of Central American migrants stops for the night in Pijijiapan, Chiapas state, Mexico, on their way toward the U.S. … — A Honduran couple, who received medical treatment for burned faces and his injured feet, rest together as a thousands-strong caravan of Central American migrants stops for the night in Pijijiapan, Chiapas state, Mexico, on their way toward the U.S. border, Thursday, Oct. 25, 2018. (AP Photo/Rebecca Blackwell)

This group also has begun to thin. Authorities say 1,740 have applied for refuge in Mexico and hundreds more have taken up offers of bus rides back to Honduras. Sickness, exhaustion and police harassment have helped whittle down their numbers.

Immigration officials appeared to be intervening more aggressively with the migrants’ movements amid the sweltering 90-degree heat.

A taxi driver in Mapastepec said he had seen immigration agents force migrant passengers out of cabs at a checkpoint.

Central American migrant Cristian pushes a carriage occupied by his daughters; Karen, 5, left, and Beiyi, 4, as they make their way to Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

Central American migrants traveling with a caravan to the U.S. climb onto to a trailer bed, hitching a ride to Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

An official from the country’s Human Rights Commission said migrants could go through if they were in vans or trucks that offered them free rides, but if they had paid they would have to get out because of the insurance regulations.

On Thursday, the long column stretched for miles along the highway. Families with young children packed sidewalks asking for donations and rides.

Honduran migrants take a bath in a river in Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

A Honduran migrant takes a bath in a river in Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

Honduran migrant Malcy Gonzales gets dressed with the help of a friend after having a bath in a river in Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

Jose Luis Ramos, a Honduran migrant traveling to the U.S. with a caravan, plays with his 3-month-old son Froilan in the central square of Mapastepec, Mexico, Wednesday, Oct. 24, 2018. (AP Photo/Rodrigo Abd)

Central American migrants Santiago Mardoqueo, 19, plays with his son Carlos, 2, take a break as they make their way to Pijijiapan, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rodrigo Abd)

Central American migrants hitch ride, as a thousands strong caravan heads north hoping to reach the U.S. border, near Mapastepec, Mexico, Thursday, Oct. 25, 2018. (AP Photo/Rebecca Blackwell)

Candy Guillermo, 37, said she had heard from others in the caravan about Trump intending to send U.S. troops to the border. A single mother of four, she was puzzled that the leader of such a powerful country would find her and the families traveling alongside her a threat.

“It surprises me because there are children here. President Trump should be more humanitarian,” Guillermo said, wiping sweat from her brow. “We only want to give our kids a better future.”