Police are investigating whether a deliberate act caused a bus fire in western Switzerland that killed at least six people and injured several others on Tuesday.

The bus caught fire around 6:25 p.m. on the main street of Kerzers, in the canton of Fribourg, about 12 miles (20 kilometers) west of Bern.

When emergency services arrived, the vehicle was engulfed in flames, and firefighters had to extinguish the blaze.

"At this stage, we have elements suggesting deliberate act by a person who was inside the bus," Frederic Papaux, a spokesperson for Fribourg police, said, adding that no other vehicle was involved.

The fire "left at least six dead and five injured, three of them seriously," Papaux said.

President expresses shock

Three injured people were taken to hospital, while two others were treated at the scene.

Authorities said panicked passengers were seen escaping the burning vehicle.

Police have launched a criminal investigation and are appealing for witnesses to come forward as they seek to determine the cause of the fire.

The bus involved was a distinctive yellow PostBus, which serves remote areas in Switzerland and is affiliated with the national postal service.

Swiss President Guy Parmelin expressed his shock and offered his condolences to the families of the victims, adding that the "circumstances are being investigated."

This is the second major incident of deadly fire in Switzerland this year after a bar in the ski resort of Crans-Montana caught fire in the early hours of January 1 killing 41 people died and injuring 115.

Edited by: Dmytro Hubenko