In this aerial view, members of law enforcement investigate a tractor-trailer in San Antonio, Texas, on Monday. AFP

At least 46 migrants were found dead on Monday in and around a tractor-trailer that was abandoned on the roadside on the outskirts of the Texas city of San Antonio.

The grim discovery was one of the worst disasters involving migrants in the United States in recent years — and came five years after a similar deadly incident in the same central Texas city, a few hours from the Mexican border.

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South Texas has long been the busiest area for illegal border crossings. migrants ride in vehicles though Border Patrol checkpoints to San Antonio, the closest major city, from which point they disperse across the United States.

A city worker at the scene on a remote back road in southwest San Antonio was alerted to the situation by a cry for help shortly before 6pm on Monday, Police Chief William McManus said. Officers arrived to find a body on the ground outside the trailer and a partially opened gate to the trailer, he said.

San Antonio Mayor Ron Nirenberg briefs media in San Antonio on Monday. AP

"The patients that we saw were hot to the touch, they were suffering from heat stroke, exhaustion," San Antonio Fire Chief Charles Hood told a news conference. "It was a refrigerated tractor-trailer but there was no visible working A/C unit on that rig."

"At this time we have processed approximately 46 bodies that have been triaged and tagged and declared deceased," San Antonio Fire Chief Charles Hood told reporters.

He said that 16 people had been transported to hospital alive and conscious — 12 adults and four children.

There were no initial details on the age or nationality of the deceased.

Temperatures in San Antonio, which is about 160 miles (250 km) from the Mexican border, swelled to a high of 103 degrees Fahrenheit (39.4 degrees Celsius) on Monday with high humidity.

Police and other first responders work the scene on Monday in San Antonio. AP

The city's Police Chief William McManus said a person who works in a nearby building heard a cry for help and came out to investigate. The worker found the trailer doors partially opened and looked inside and found a number of dead bodies.

McManus said this was the largest incident of its kind in the city and said three people were in custody following the incident, though their involvement is not yet clear.

A spokesperson for US Immigration and Customs Enforcement (ICE) said that its Homeland Security Investigations division was investigating "an alleged human smuggling event" in coordination with local police.