At least six bus passengers were killed and dozens more injured when a bomb planted on a parked motorbike exploded in Pakistan's province of Baluchistan, officials said.

The motorbike fitted with explosives had been left on the road linking the provincial capital Quetta with Iran, Naseebullah Bazai, a senior government official, told the AFP news agency on Monday.

The incident happened near Dreengar town in Mastung district, about 40km south of Quetta.

Target 'not clear'

Mastung administration chief Shah Irfan confirmed the casualties and said several passengers with serious injuries had been rushed to hospital.

"The target of the attack was not immediately clear," he said.

Local officials said a passenger bus carrying pilgrims to Shia holy sites in neighbouring Iran had passed the motorbike shortly before the blast.

"We are investigating if the target was the pilgrims' bus," Bazai said.

The attack came after 15 people were killed and 30 wounded in a bomb explosion outside a madrassa, a religious school, in Quetta on Thursday.

There was no claim of responsibility for the latest incident, but oil- and gas-rich Baluchistan, which borders Iran and Afghanistan, suffers from sectarian violence between majority Sunni and minority Shia Muslim communities and a separatist uprising.

Hundreds of civilians have been killed since Baluch rebels rose up in 2004 against the federal Pakistani government, demanding political autonomy and a greater share of profits from the resources in the region.