XALAPA, Mexico (AFP) - A bus burst into flames after slamming into the back of a parked trailer-truck in south-eastern Mexico, killing at least 25 people and injuring four others, an official said Sunday.

The death toll could rise to 33 but the fire made it difficult to identify all the bodies, Veracruz Governor Javier Duarte told Milenio television.

"The trailer-truck was on the shoulder of the highway at kilometer 135... when the bus hit the back of the trailer, which caused this regrettable accident," he said. "In addition to crashing, the bus caught fire, which makes the investigative work to identify bodies more difficult."

The accident took place between 6.30am GMT and 7am GMT as the bus took passengers from the southeastern Tabasco state city of Villahermosa to Mexico City.

The tragedy occurred as Mexicans are set to begin Holy Week in the predominantly Roman Catholic country.