Islamabad (PTI): At least 16 people, including four children and four policemen, were killed and several others injured on Thursday when a roadside bomb went off in Pakistan's restive northwest, while a prison van and a school bus were passing by.

The van was bringing prisoners from a court to jail when it was attacked with a remote-controlled bomb in Dir Bala area of North West Frontier Province, state-run PTV reported. A school bus passing through the area was also hit by the blast.

Sixteen people, including four school children and several prisoners and policemen, were killed.

Local residents took the injured to nearby hospitals in private vehicles. No group claimed responsibility for the attack.

The people in Dir district had recently formed a 'lashkar' or militia to protect themselves against the local Taliban forcing militants to leave the area.

Earlier, two Chinese engineers were kidnapped by militants from Dir last month and later taken to the restive Swat valley.

Taliban spokesman Muslim Khan had demanded the release of around 130 militants as a condition to release the abducted Chinese men.

Meanwhile, in a separate incident in southwestern Balochistan province's capital Quetta, two armed men riding a motorcycle lobbed a grenade at a car parked outside a college.

The blast damaged the vehicle though there were no casualties, police officials said.