Tufail Ahmed says over 20 people were wounded in the attack Sunday in Baluchistan province's Mastung district.
The blast completely destroyed the bus that was hit and damaged a second bus carrying Shiites that was close by.
An eyewitness who was traveling in the second bus told Geo TV that the pilgrims were headed to neighboring Iran, a majority Shiite country that is a popular religious tourism destination. The eyewitness did not provide his name.
Pakistan has experienced a spike in killings over the last year by radical Sunni Muslims targeting Shiites who they consider heretics, especially in Baluchistan.
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