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 of mobile phones aggravated the fire leading to the tragic event, CNN News18 quotes Kurnool Superintendent of Police Vikrant Patel as stating.

Meanwhile, P Venkataraman, the director-general of the Andhra Pradesh Fire Services Department, reported that electrical batteries used for the air conditioning system of the bus also exploded, contributing to the fire's severity.

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

The smartphones, valued at 4.6 million Indian rupees (£39,361; $52,377), were being transported from Hyderabad to an e-commerce company in Bengaluru, according to broadcaster NDTV.

Most smartphones contain lithium-ion batteries, which are known to pose fire risks when damaged due to thermal runaway—a rapid uncontrolled temperature increase that can spread to nearby batteries.

Authorities have launched an investigation into possible negligent driving related to the crash, with reports indicating that the bus driver fled the scene and the motorcyclist involved is among the deceased.

Buses are a common mode of transport in India, but operators often disregard safety regulations and overcrowd vehicles. The country recorded the highest road accident death tolls globally last year.