At least 44 people have died after a bus overloaded with passengers came off a mountain road in northern India, plunging to the bottom of a 500ft gorge.

The crash in the Kullu district of Himachal Pradesh occurred on Thursday and 25 people were declared dead at the scene, while another 19 succumbed to injuries overnight in hospital.

More than a dozen others were being treated in hospital in Kullu town, said police officer Amar Singh.

The bus had been on its way to Gada Gushaini, about a 60km ride from Kullu. Police said a rescue operation had been launched immediately after the crash, while TV images showed the mangled remains of the bus and people forming a human chain across the gorge floor to reach survivors.

Shalini Agnihotri, the Kullu police chief, said the cause of the accident was being investigated.

But the Press Trust of India news agency quoted a government official, Sheetal Kumar, as saying that it appeared the bus had too many people - as many as 70 - on board at the time. Reckless driving may also have been a factor, he said.

Prime minister Narendra Modi said he “hope[s] the injured recover soon” and offered his “condolences to the families of those who lost their lives”.

Mr Modi said the Himachal Pradesh government was providing “all possible assistance that is required”. Most of the victims were local to Kullu district, and the district administration announced an immediate relief fund of Rs50,000 (£570) each to the families of those killed and injured.

Around 150,000 people die every year on India's roads, according to government data, with crashes often caused by either crowded vehicles, bad road and vehicle conditions or reckless driving. Only drivers and front-seat passengers are required to wear a seatbelt by law.

Sitting at the foothills of the Himalayas, Himachal Pradesh is particularly prone to deadly crashes. The state’s chief minister Jai Ram Thakur, ordered an inquiry into Thursday’s crash.

Last September, 55 people died when a bus carrying pilgrims from a Hindu temple in the hills of south India plunged off a road.