Two men on a motorbike attacked a vehicle carrying four Shiite vegetable sellers as it left the provincial capital Quetta.
"The local police escort the Shiite vegetable sellers daily... one of the vehicles today fell victim to sniper fire after it got separated from the rest of the escort," senior local police official Abdul Razzaq Cheema told AFP.
He said the gunmen killed three Shiites from the Hazara ethnic minority as well as the van driver and a passer-by.
One Shiite vegetable seller suffered several bullet wounds but survived.
Another senior local police official, Naseeb Ullah, confirmed the incident and casualties.
There was no immediate claim of responsibility. Shiites make up roughly 20 per cent of Pakistan's 200 million people.
Attacks by Sunni hardliners against them, along with other sectarian violence, have claimed thousands of lives in the country over the past decade.
The southwestern province of Balochistan, which borders Iran and Afghanistan, has ample oil and gas resources but is afflicted by Islamist militancy, sectarian violence and a separatist insurgency.
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