A bus and a car collided in a highway tunnel in eastern Switzerland Saturday, killing at least six people, authorities said.

Fire broke out in the Viamala tunnel following the accident on the A13 highway in the canton of Graubuenden, and heavy smoke was hindering rescuers, police said.

Police and rescue forces were mobilized, and a rescue helicopter was sent to the scene.

Several injured people were able to get out of the tunnel, police said.

The cause of the accident, as well as the number of people who were in the two vehicles, was unclear.

Police said the accident happened at 1:15 p.m. on the highway from Chur to the San-Bernardino tunnel, and the vehicles were partially burnt out.

The Viamala tunnel is 742 meters long. Police blocked the highway in both directions.

The worst accident in a Swiss highway tunnel happened in 2001, when the collision of two lorries killed 11 people in the Gotthard tunnel.