“The suicide bomber was riding a motorbike and hit the truck from behind,” senior police official Abdul Hai Aamir told AFP.
The incident took place near Dhadar, the main town of Kachhi district, about 120 kilometres (75 miles) southeast of Quetta in Balochistan.
Photos of the aftermath showed the police truck upside down on the road with its windows shattered.
Mehmood Notezai, police chief for Kachhi district, told AFP the officers were returning from a week-long cattle show where they had been providing security.
There has been no claim of responsiblity for the attack.
“Terrorism in Balochistan is part of a nefarious agenda to destabilise the country,” Prime Minister Shehbaz Sharif said in a statement released by his office.
Last month five people died when a TTP suicide squad stormed a police compound in the port city of Karachi.
It came just weeks after a bomb blast at a police mosque in the northwestern city of Peshawar killed more than 80 officers — an attack claimed by a group sometimes affiliated with the TTP.
The country is facing overlapping political, economic and environmental crises, as well as a worsening security situation during which the army and police have been increasingly targeted.
Balochistan is the largest, least populous and poorest province in Pakistan.
It has abundant natural resources, but locals have long harboured resentment, claiming they do not receive a fair share of its riches.
Tensions have been stoked further by a flood of Chinese investment under Beijing’s Belt and Road Initiative, which locals say has not reached them.
China is investing in the area under a $54-billion project known as the China-Pakistan Economic Corridor, upgrading infrastructure, power and transport links between its far-western Xinjiang region and Pakistan’s Gwadar port.
