Unnao: Six people, including a police sub-inspector, were killed and 21 others injured when a double-decker AC bus overturned on the Lucknow-Agra Expressway in Uttar Pradesh's Unnao district early Tuesday, police said.

The accident took place at 5.15 am near Nibha Kheda village when the bus, travelling from Delhi to Bihar with around 50 passengers on board, went out of control and overturned, they said.

According to preliminary investigation, the driver allegedly lost control of the vehicle after dozing off at the wheel.

Eyewitnesses said the bus was speeding when it suddenly hit the divider and overturned. Following the impact, the vehicle got suspended on the roadside railing, causing several passengers to fall out through the windows.

Read More: Maharashtra SUV crash: Eight killed after vehicle falls into gorge in Raigad

Police said efforts were underway to identify the deceased.

Soon after the accident, police and rescue teams rushed to the spot and shifted the injured to nearby government hospitals.

The critically injured passengers, including three police personnel, were referred to the trauma centre in Lucknow, while others are undergoing treatment at the district hospital.