Mexico's Foreign Minister Marcelo Ebrard called the suffocation of the migrants in San Antonio the "tragedy in Texas" on Twitter and said the local consulate was on the way to the scene, though the nationalities of the victims had not been confirmed.Â
A US law enforcement official said there were more than 40 people dead on Monday without providing additional details.
The truck was found next to railway tracks in a remote area on the city's southern outskirts, San Antonio's KSAT television reported.
There have been a record number of migrant crossings at the US-Mexico border in recent months, which has sparked criticisms of the immigration policies of US President Joe Biden, a Democrat.
Texas Governor Greg Abbott, a Republican, placed the blame for the deaths on Biden's policies on Twitter, saying they "show the deadly consequences of his refusal to enforce the law".
The San Antonio police did not immediately respond to a request for comment.
Temperatures in San Antonio, which is about 250 kilometres from the Mexican border, swelled to a high of 39.4C on Monday with high humidity.
In July 2017, 10 migrants died after being transported in a tractor-trailer that was discovered by San Antonio police in a store car park.
The driver, James Matthew Bradley, Jr, was sentenced the following year to life in prison for his role in the smuggling operation.
