Lucknow | Five people, including two children, were killed when a private bus caught fire in the Mohanlalganj area here early Thursday, police said.

They said the incident occurred around 5 am on Kisan Path, a major roadway on the outskirts of Lucknow.

According to sources, the bus was carrying 80 passengers.

Assistant Commissioner of Police (Mohanlalganj) Rajneesh Verma said the bus was en route to Delhi from Begusarai in Bihar when it suddenly caught fire.

"Initial investigation suggests that the fire may have started due to a short circuit in the bus' gearbox," Verma told PTI.

Most of the passengers were asleep when the fire started. It spread rapidly and five passengers -- two women, two children and a man -- were unable to get out in time and died. Several other passengers sustained injuries and have been taken to a hospital for treatment, police said.

According to eyewitnesses, the flames were visible from almost a kilometre away. The bus did not come to a halt immediately after the fire broke out, the eyewitnesses said.

Firefighters reached the spot promptly and managed to control the blaze within half an hour.

Police found that the emergency exit door of the bus failed to open, which led to passengers sitting in the rear getting trapped inside.

According to authorities, a probe to determine the exact cause of the fire is underway.