City officials believe the dead were migrants.

Fire officials said they found "stacks of bodies" and confirmed that 16 people, including four children, were taken to hospital and are "hopeful to survive".

In a news conference, San Antonio's fire chief, Charles Hood, said that the truck was refrigerated but there was no water or working air conditioning, adding the patients were hot to the touch, and suffering from heat exhaustion.

"We are not supposed to open up a truck and see stacks of bodies. Nobody comes to work for that," he said.

No children are among the dead in the truck, which was found next to railroad tracks in the city's Southwest Side neighbourhood.

The San Antonio police chief, William McManus, confirmed that three people are in custody and a federal investigation has been launched.

Mr McManus said a person who works in a nearby building heard a cry for help and came out to investigate.

The worker found the lorry doors partially opened and looked inside to find a number of bodies.

He added this was the largest incident of this kind in the city.

Texas governor Greg Abbott, a Republican politician, tweeted that "these deaths are on Biden", adding: "They are the result of his deadly open border policies."

A spokesperson for U.S. Immigration and Customs Enforcement said its Homeland Security Investigations division was investigating "an alleged human smuggling event" in coordination with local police.

San Antonio's mayor Ron Nirenberg said that this "is nothing short of a human tragedy", adding: "Our job is not to ask why. Our job is to ask how we can help."

The Secretary of Foreign Affairs for Mexico, Marcel Ebrard, said that a consul is on its way to the neighbourhood and that at least two of those in hospital were from Guatemala.

"Tragedy in Texas. Reportedly suffocated in trailer box," he wrote on Twitter.

"The trailer has EU plates, superimposed, to circulate without revision. Very likely authorship of traffickers."

Archbishop Gustavo, a Catholic priest in San Antonio lamented the tragedy so soon after the Uvalde shooting, writing on Twitter: "Lord have mercy on them. They hoped for a better life. Lord after Uvalde and now this, help us! We need you! So many people suffering. God, God, God."

Although no official cause of death has been recorded, temperatures in San Antonio, which is about 160 miles (250 km) from the Mexican border, hit a high of 103F (39.4C) on Monday with high humidity.