At least 44 people are feared dead after a private bus swerved off a mountain road and plunged into a deep gorge in northern India's Himachal Pradesh state, according to a government official.

The rescue operation is ongoing and casualties could increase, according to Rohan Chand Thakur, deputy commissioner of Shimla district, where the accident occurred on Wednesday.

"Our officials, workers and medical teams have rushed to the spot for the rescue work. It’s not possible now to say the details," Thakur said.

The cause of the accident is not yet known.

The Indian Express daily reported that the bus skidded off the road and fell nearly 250 metres downhill.

Among the casualties are 10 women and three children.

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Only two people, including the ticket checker, survived the crash, according to Thakur.

He said that rescue teams were working to recover the wreckage from the gorge.

Driver fatigue, negligence, poor-quality roads and vehicle maintenance are the usual causes of such accidents in India.

Police figures show India has the world's highest road accident death toll, with more than 110,000 people dying each year in crashes.