The bodies of 46 dead migrants have been discovered inside a tractor-trailer in San Antonio, Texas, in one of the most deadly recent incidents of human smuggling along the US-Mexico border.

A San Antonio Fire Department official said they found “stacks of bodies” and no signs of water in the truck, which was found on Monday next to railway tracks in a remote area on the city’s southern outskirts.

Sixteen other people found inside the trailer were transported to hospitals for heat stroke and exhaustion. They included four minors, although no children were among the dead, the department said.

“It’s probably close [to] 100 in the truck,” a law enforcement source told the San Antonio Express-News, adding that “about half” of the group were likely dead.

“A worker, who works in one of the buildings up behind me, heard a cry for help. Came out to investigate and found a trailer with the doors partially open and found a number of deceased individuals inside,” San Antonio Police Chief William McManus said later.

The discovery prompted an enormous police response, with first responders seen walking along the tracks with thermal imaging cameras, apparently searching for more people.

Police have arrested three people, although Chief McManus warned they did not “absolutely know” if they were connected with the tragic discovery.

San Antonio Fire Chief Charles Hood said about 60 firefighters were involved in the rescue operation, which he described as a “mass casualty with triage”. Ten medic units transported the 16 patients to hospital.

“The patients that we saw were hot to touch. They were suffering from heatstroke, heat exhaustion. No signs of water in the vehicle. It was a refrigerated tractor-trailer, but there was no visible working AC unit on that rig,” he said.

Mr Hood said those still alive were too weak to help themselves out of the truck.

Temperatures in San Antonio, which is about 250 kilometres from the Mexican border, rose as high as 39.4 degrees on Monday with high humidity.

San Antonio police were also searching for the truck’s driver, who had apparently abandoned the vehicle prior to its discovery, The New York Times reported.

A spokeswoman for US Customs and Border Protection said federal agents had arrived at the site by Monday night.

There have been a record number of migrant crossings at the US-Mexico border in recent months, sparking 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 Mr Biden’s policies, saying on Twitter that they showed “the deadly consequences of his refusal to enforce the law”.

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A KSAT-12 reporter called the situation “a possible human smuggling case” in a tweet as she headed to the scene.

San Antonio Mayor Ron Nirenberg said the deaths were “nothing short of a horrific human tragedy.”

Mexican Foreign Minister Marcelo Ebrard called the migrants’ suffocation the “tragedy in Texas” on Twitter and said consular officials would go to the hospitals where victims had been taken to help “however possible”.

The Honduran foreign ministry said the country’s consulates in Houston and Dallas would investigate the incident.

Mr Ebrard said two Guatemalans were hospitalised and Guatemala’s foreign ministry said on Twitter that consular officials were going to the hospital “to verify if there are two Guatemalan minors there and what condition they are in”.

Rubén Minutti Zanatta, Mexico’s consul-general in San Antonio, said in a tweet he was waiting outside the city late on Monday for federal authorities to release the nationalities of the deceased.

The I-35 highway near where the truck was found runs through San Antonio from the Mexican border and is a popular smuggling corridor, according to Jack Staton, a former senior official with ICE’s investigative unit who retired in December.

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.

-with AAP