The incident, the latest in a string of attacks, occurred in the Loralai district of mineral-rich Baluchistan province.
Around 40 militants armed with rocket launchers and other heavy weapons attacked a security checkpoint run by the Frontier Constabulary, a paramilitary force tasked with maintaining security, a senior military official said on condition of anonymity.
Many of the attackers were wounded by the paramilitary's retaliatory fire, he added.
No group has claimed responsibility for the attack.
Baluchistan, which covers more than 40 percent of Pakistan's territory, has been beset by violence for decades. Both Taliban and separatist Baluch militants are active, with the latter seeking independence from Pakistan.