The attack took place at Dringhar village on the Pakistan-Iran highway some 60 kilometres (37 miles) west of Quetta, Balochistan'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 due to the extent of carnage caused.

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.

Two people were killed in a similar attack on January 1 when a bomb targeted a bus carrying pilgrims.

Nobody claimed responsibility for that attack.

(APP)