Two bogies of the train derailed following the blast which took place near Bahawalpur in Punjab province, local Geo TV reported.

It said six people were killed and several injured in the blast. The dead included a woman and three children. The injured, said to be in critical condition, were rushed to Victoris Hospital in Bahawalpur while rescue teams have also been sent to the accident site, the report said.

Citing police, it said the explosives were planted under the railway lines which damaged two bogies. Several people were still trapped in the bogies while the train service had been suspended on the track after the blast.

The incident came nearly a week after a massive blast at the five-star Marriott Hotel that killed at least 53 people and wounded 270.

Senior police and railway officials rushed to the site of the accident and monitored rescue and relief activities.

People had to cut through the twisted metal of the coaches to pull out the injured.

President Asif Ali Zardari and Prime Minister Yousuf Raza Gilani condemned the blast and the killing of innocent people.

In a message, Zardari said such acts would not deter the government's resolve to fight terrorism and extremism.

He said the perpetrators of such heinous crimes would be brought to justice.