The men opened fire at the NATO trucks in Nach area of Khuzdar district.
They set the trucks on fire and fled.
No one was injured in the attack, officials of the Levies militia said.
The trucks carrying supplies for NATO forces in Afghanistan were on their way to the Chaman border crossing.
Levies personnel cordoned off the area and launched a search to track down the attackers.
Pakistan ended a seven-month blockade of supply routes to Afghanistan earlier this month after the US apologised for a NATO air strike that killed 24 Pakistani soldiers last November.
The US and Pakistan have also signed a memorandum of understanding on the transit of NATO supplies to Afghanistan.
There have been several attacks on NATO vehicles after the supply routes were reopened by Pakistan.
