QUETTA, Pakistan (AFP) - At least 22 Shi'ite pilgrims returning from Iran were killed and 20 others wounded when a bomb hit a bus in the south-west Pakistan province of Baluchistan Tuesday, officials said.

The attack took place at Dringhar village on the Pakistan-Iran highway some 60km west of Quetta, Baluchistan's capital.

"At least 22 people were killed and 20 others were wounded," local government official Shafqat Shahwani told AFP.

He said 51 passengers had been on board at the time of the blast and that nine passengers were still unaccounted for.

Provincial Home Secretary Asad Gilani said it was not immediately clear what type of bomb was used or whether it was a suicide attack.

He said that two buses had been travelling together with security vehicles provided by the government and that the bomb struck one of the buses.

Mr Shahwani said that bomb disposal officials suspect the bomb was planted on the roadside and detonated remotely, but he did not rule out the possibility of a suicide attack.

He added that the driver of the bus told the authorities that he did not see any car hitting his vehicle, but felt a sudden blast followed by the cries of women and children amid the destruction.

"We have declared an emergency at hospital and doctors and paramedics are trying to save the wounded," Mr Shahwani told AFP.

Pakistani President Mamnoon Hussain and Prime Minister Nawaz Sharif strongly condemned the bombing in separate messages.

Two people were killed in a similar attack on Jan 1 when a bomb targeted a bus carrying Shi'ite pilgrims near Quetta.

Nobody claimed responsibility for that attack but there has been a rise in sectarian violence in Pakistan after several deadly clashes between Sunni and Shi'ite Muslim groups near Islamabad in November last year.

Shi'ites make up around 20 per cent of Pakistan's population, which is largely Sunni Muslim.

Oil and gas-rich Baluchistan has been badly hit by a decade-long Baluch separatist insurgency and sectarian violence, mainly targeting Shiite Muslims from the Hazara ethnic community.

Two huge bomb attacks in Quetta in January and February last year targeting Shi'ites from the Hazara ethnic community killed nearly 200 people.