A Reuters witness heard desperate cries and sobs from the survivors as Mexican officials rushed to the scene.
Reuters images show a white trailer flipped onto its side on a highway, with some people splayed out on tarps on the ground for medical care. Images also show rows of what appear to be bodies of the accident victims wrapped in white cloths.
A video of the aftermath broadcast on social media showed a woman holding a child wailing in her lap, both bathed in blood. Another video showed a man curled up in pain inside the destroyed trailer, hardly moving as helpers pulled out bodies.
US president Joe Biden’s administration has also urged migrants not to leave their homelands for the United States
The fatalities included men, women and children, Chiapas civil protection said.
President Andres Manuel Lopez Obrador on Twitter expressed his sorrow at the “very painful” incident.
The accident also injured several dozen people, who were taken to hospitals in the area, authorities said.
Migrants fleeing poverty and violence in Central America typically trek through Mexico to reach the U.S. border, and sometimes cram into large trucks organized by smugglers in extremely dangerous conditions.
“This shows us that irregular migration is not the best way,” Kevin Lopez, a spokesman for Guatemala’s presidency, told Milenio television.
He did not know how many Guatemalan victims there were.
El Salvador’s foreign minister Alexandra Hill said her government was working to see if Salvadorans had died.
Mexico’s national migration institute said it would offer lodging and humanitarian visas to the survivors, and Chiapas Governor Rutilio Escandon said those responsible for the accident would be held to account.
Officials in Mexico routinely come across migrants packed into trailers, including 600 people found hidden in the back of two trucks in eastern Mexico last month.
Mexican authorities in Chiapas have attempted to persuade migrants to not form caravans to walk the thousands of miles to the US border, and have begun transporting people from the southern city of Tapachula to other regions of the country.
US president Joe Biden’s administration has also urged migrants not to leave their homelands for the United States.
Critics have said tougher policies lead migrants to seek out human smugglers, putting their lives at risk.
“(Authorities) generate smuggled migration that generates billions of dollars in profits,” said migrant activist Ruben Figueroa.