A truck slammed into a bus bringing schoolchildren home from an outing in southern Pakistan, killing at least 17 pupils and three adults, police have said.

The collision occurred on Wednesday in southern Sindh province, about 300km north of the provincial capital, Karachi.

"It was a head-on collision when the truck slammed into the school van on a single track dirt road. Apparently, speeding and recklessness resulted in the accident," Abdul Rahim Gopang, a police officer, said by telephone from near the accident.

Television channels showed footage of parents running into a hospital in the nearby town of Nawab Shah, frantically looking for their children, aged 10 to 16.

"All our dreams have been shattered. How can I live without my child," a weeping Muhammad Akram, who lost his only son in the accident, said by telephone from the hospital.

Eleven children were injured and being treated in hospital, said Sindh's Information Minister, Sharjeel Inam Memon.

Appeals for blood donations for the injured were issued from the loudspeakers of mosques, local television channels said.

Source: Agencies