"At least 40 people have died. Their body parts are scattered on both sides of the train track," said Khalid Ahmed, a police officer based in the central Punjab town of Malikwal near the accident site.

According to one police official quoted by Reuters, the death toll was at least 40.

The east-bound train rammed into the 44-seater bus as it drove across the railway tracks at around 8:25 am (0325 GMT), Ahmed said, killing most of the bus passengers. Eight others were critically wounded.

"The bus was on the railway line at the crossing," police officer Mohammad Naeem told AFP by telephone from the twin town of Mandi Bahaudin.

"No one was hurt on the train and the train did not derail. There is nothing left of the bus," Ahmed said.

The crash happened near Malikwal, a town about 150 km south of the capital Islamabad, as the bus was attempting to cross the railway track, local police chief Waqar Haider said.

He said all those killed had been on board the bus.

The BBC's Zaffar Abbas in Islamabad reports that nearly all the train passengers escaped unharmed in the crash, which happened early on Saturday between the towns of Lalamusa and Surghuda in Punjab province.

But nearly all the bus passengers were killed - at least 20 of them on the spot - and the impact reduced parts of the vehicle to twisted metal.

A number of people were injured, six critically.

The police chief said the railway crossing did not have a gate, nor any warning lights to alert vehicles to oncoming trains.

All train services have now been suspended on the line and a full investigation is under way.

A spokesman for the Pakistan Railways said the crash appeared to have been caused by the bus driver's negligence.

Local villagers say it is the third collision at the unmanned level crossing in less than two years, but despite repeated demands, the railway authorities had failed to install a gate.

Many railway crossings in the Punjab, Pakistan's most populous province, are without gates and warning signals.