Officials in Pakistan say at least 22 Shiite pilgrims have been killed and 20 others injured in a bomb attack on a bus in the south-west province of Baluchistan.

The bus was returning from a pilgrimage to Iran when it was blown up 60 kilometres west of Quetta, the provincial capital of Baluchistan.

Officials say 51 passengers were on board at the time and nine are still unaccounted for due to the extent of the damage.

Authorities say security vehicles provided by the government were travelling with the bus.

It is not clear whether it was a suicide attack.

Shiites make up about 20 per cent of Pakistan's population, which is largely Sunni Muslim.

The bombing is the latest in a series of deadly attacks across Pakistan.

On Sunday in Khyber-Pakhtunkhwa, the Pakistani Taliban blew up a military convoy killing more than 20 soldiers.

On Monday, a suicide bomber killed at least 13 people near Pakistan's army headquarters in Rawalpindi.