Two bombs have struck a passenger train minutes apart in south-west Pakistan, killing at least six people and wounding 19, officials said.

The first bomb exploded on the train and the second device, planted on the track, struck the moving carriages near the village of Mach in Baluchistan province, according to railways official Tufail Ahmad and police spokesman Mohammad Sajid.

Mr Ahmad said the train was heading to the garrison city of Rawalpindi in the eastern province of Punjab from Quetta, the provincial capital of Baluchistan. He said the casualties were taken to nearby hospitals and some of the wounded were in critical condition.

Initially officials said one bomb went off and three people were reported dead, but three more later died of their injuries, according to Mr Sajid, who examined the damaged train and track.

Baluchistan has long been the centre of a low-level insurgency by nationalist groups that demand more autonomy and a greater share in the region's natural resources.

The government says it is trying to improve people's welfare in the province and bring the separatists into mainstream politics.

Railways minister Khawaja Saad Rafique denounced the bombing as "an act of terrorism" and said authorities were trying to determine how the bomb was planted on the train.

Prime minister Nawaz Sharif condemned the attack and ordered authorities to find and arrest those behind it.