A bus has swerved off a mountain road and plunged into a deep ravine in northern India, killing at least 44 people, an official said Wednesday.

The bus with 56 passengers on board plunged into a river in northern Himachal Pradesh state around 115 kilometres from the capital Shimla.

“A preliminary report shows that a private bus fell into a gorge and has killed more than 40 people. The latest report given to me shows 44 people have been killed,” Rohan Chand Thakur, district commissioner of Shimla, told AFP.

Police rushed to the scene where rescue efforts were underway, he added. It was not immediately clear what caused the accident, or whether any of the survivors were injured.

India has some of the world's deadliest roads with more than 150,000 fatalities annually due to poor roads, badly maintained vehicles and reckless driving.

In February, 16 people were killed and more than 50 injured when a truck carrying churchgoers overturned in a hilly region of northeast India.

Survivors said the driver was manoeuvring recklessly and lost control of the vehicle in the rugged terrain. A month earlier 13 people ─ mostly children aged between seven and 14 ─ were killed when a school bus collided with a truck in northern India.