The gunmen opened fire after forcing the convoy of eight oil tankers to stop in an area 90 kilometers (55 miles) southwest of Quetta, the capital of Balochistan province, police official Abdul Qadir said.
The militants killed one of the truck's drivers and set the tankers on fire before they escaped, Qadir said.
Fuel and supply trucks contracted to supply NATO have been left stranded in Pakistan by Islamabad's decision to block its two NATO supply routes to Afghanistan.
Pakistan closed the routes to protest a NATO airstrike that killed 24 Pakistani soldiers last month.
The airstrike plunged Islamabad and Washington into one of their worst diplomatic crises ever.
In a television interview on Sunday, Pakistan's Prime Minister Yousuf Raza Gilani said the blockade of Pakistan's NATO supply routes could continue for weeks until "new rules of engagement" were established with Washington.
The blockade leaves stranded supply trucks vulnerable to militants in Pakistan who have increasingly targeted the convoys in an effort to undermine the NATO mission in Afghanistan.
Sunday's attack was the second of its kind in four days.
On Thursday, militants armed with rocket-propelled grenades and automatic weapons attacked and destroyed more than 20 NATO-bound oil tankers just outside Quetta.
Roughly 40% of supplies for NATO operations in Afghanistan traveled through Pakistan before the supply routes were shut down