All appeared unharmed and the trailer had fans and ventilation ports, officials said. The driver's whereabouts are unknown.

Fleeing poverty and violence in Central America, many undocumented migrants end up paying huge sums of cash to people-smugglers to get them across the US border. Last year, 53 migrants were found dead in an abandoned truck trailer in the US state of Texas.

A total of 343 people were found in the vehicle on Sunday night, Mexico's National Migration Institute (INM) said.

They were found on a common route used to take migrants to the US border.

The migrants wore colour-coded bracelets to identify them as smugglers' clients, officials said.

A photo distributed by INM showed plastic bottles and rubbish strewn across the floor of the trailer.

The migrants will be cared for by Mexican social services until their status is decided. Border patrol data obtained by CBS News in October shows that at least 853 migrants died trying to cross the US-Mexico border unlawfully in the previous 12 months.

That made the fiscal year of 2022 the deadliest year for migrants recorded by the US government, far exceeding the previous record of 546 migrant deaths in 2021. US Border Patrol apprehended a record 2.2 million migrants at the south-west border in the same period