The district disaster control room said as of now five deaths are confirmed but the number might go up as there were 40 passengers in the bus at the time of the accident.
"The bus was headed towards Ahmednagar," District Disaster Control Officer Jaideep Visave said.
The injured are being rushed to hospitals at Murbad and Tokawade.
According to unconfirmed reports, the mishap occurred when driver of the bus lost control while negotiating a sharp turn.
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Malshej Ghat is a famous picnic spot located in the Sahyadri mountain range, situated on the border of Thane and Ahmednagar districts.
Many ambulances were rushed to the spot for shifting the injured, police said.
Meanwhile, MSRTC GM (traffic) Suryakant Ambawdekar said erratic network coverage was hampering communication with officials at the spot.
"As per initial information available with me, a truck collided with the bus knocking it off into the valley," he said.