Two suspected undocumented immigrants suffocated to death aboard a freight train and 10 others in need of medical care were taken by helicopter or ambulance to hospital in south Texas, US police say.

Federal investigators were looking into the possibility of human smuggling, investigators say.
Federal investigators were looking into the possibility of human smuggling, investigators say. (AP Archive)

Two migrants have been found dead, and at least 10 were hospitalised after police in southern Texas received a call that they were "suffocating" in a freight train car travelling near the US-Mexico border.

The Uvalde Police Department said Border Patrol was informed of the phone call on Friday and able to stop the train.

About 15 migrants were found inside, according to a statement from the department.

University Health in San Antonio tweeted that it had received two male patients, one in critical condition and one in serious condition.

Union Pacific railroad said in a statement that the people were found in two train cars: 12 in a shipping container and three in a hopper car.

The two people who died were in the shipping container, the statement said.

Uvalde Police Chief Daniel Rodriguez told the San Antonio-Express News that dispatchers received a 911 call at about 3:50 pm from an unknown person seeking help.

The train was stopped near the town of Knippa, which is less than 161 kilometres from the southern border.

"We’re still trying to determine if it was from someone inside the car," Rodriguez said. "We're assuming it was from inside one of the cars."

Federal investigators were looking into the possibility of human smuggling, investigators with the US Department of Homeland Security said.

The Mexican consulate in Eagle Pass, Texas, said on Twitter it was aware of the incident and in communication with US officials to determine if any of the victims were Mexican.

Homeland Security had yet to determine the nationalities of the victims or whether any families or children were among them, a department official with knowledge of the matter said, speaking on condition of anonymity.

READ MORE: Texas migrant tragedy revives debate on Trump policies

'Diminished sense of vigilance'

Last summer, more than 50 migrants died after dozens of people were found in the back of a sweltering tractor-trailer abandoned on the outskirts of San Antonio.

The tragedy was the nation's deadliest smuggling episode on America's southern border, which led to officials vowing to step up policing efforts.

Two Americans have been indicted in federal court in that case and could face the death penalty if convicted. Two Mexicans have been charged with lesser crimes.

Migrants routinely travel through Uvalde, leading to high-speed vehicle pursuits that put schools in the area on lockdown.

After the Robb Elementary School massacre in Uvalde last May, when a gunman killed 19 children and two teachers, Texas lawmakers concluded in a report that the frequency of the lockdowns may have led to a "diminished sense of vigilance" about security.

Uvalde police said that Union Pacific railroad would lead the investigation. A spokesperson for Union Pacific did not immediately comment Friday night.

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Source: TRTWorld and agencies