A bus fire that killed at least 20 people in southern India was intensified by hundreds of smartphones being onboard, local reports say, citing forensics officials.

Flames rapidly engulfed the Bangalore-bound bus after it collided with a motorcycle that ruptured its fuel tank and triggered an explosion early on Friday.

Eyewitnesses said the approximately 40 passengers had struggled to escape as locals rushed to pull out survivors.

Forensic experts have now told local media that the bus had been carrying a shipment of 234 mobile phones, and that the lithium-ion batteries they contained likely exacerbated the fire once they ruptured.

"The bus batteries, presence of flammable furnishings in the bus and the cargo consisting of cell phones aggravated the fire leading to the tragic event," CNN News18 quotes Kurnool Superintendent of Police Vikrant Patel as saying.

Meanwhile, P Venkataraman, the director-general of the Andhra Pradesh Fire Services Department, said electrical batteries used for the air conditioning system of the bus also exploded and made the fire worse.

"We saw bones and ash dropping through the melted sheets," he told local media.

The smartphones were worth 4.6m Indian rupees (£39,361; $52,377) and were being sent from Hyderabad to an e-commerce company in Bengaluru, broadcaster NDTV reports.

Most smartphones have lithium-ion batteries, which can pose a fire risk when damaged.

This is because they can experience thermal runaway - uncontrollable temperature increases - which is hard to stop with conventional firefighting means. It can also spread to other batteries in close proximity.

Authorities have opened an investigation into possible negligent driving over the crash. The bus driver fled the scene. The motorcyclist is said to be among the dead.

Buses are a common mode of transport in India, but operators frequently ignore safety rules and overcrowd vehicles, while the nation had the highest road accident death tolls in the world last year.