Eighteen migrant labourers sleeping on a highway in northern India after their bus broke down died when a truck rammed into the vehicle, police said Wednesday.

At least 19 others were injured in the accident in Uttar Pradesh state, a senior police officer told reporters.

Most of the passengers were returning home to the eastern state of Bihar after working in the states of Punjab or Haryana.

The passengers got off the bus after its axle shaft broke and were sleeping next to it when a truck crashed into it from behind.

Rescue workers retrieved some of the bodies from under the mangled double-decker bus.

"The district administration and the police have launched a probe and we are ensuring that the wounded receive the best medical treatment that's available," said police officer Satya Narayan Sabat.

India's vast network of roads is poorly maintained and notoriously dangerous.

About 150,000 people are killed each year in traffic accidents in India, according to the government.

Among the main factors contributing to the high number of fatalities are excessive speeding and people not using seatbelts or wearing crash helmets.