QUETTA, Pakistan (AFP) - Gunmen in Pakistan on Monday set ablaze five trucks carrying NATO equipment out of Afghanistan as the international military alliance winds down its combat mission there, officials said.

Four masked gunmen on two motorbikes opened fire at the vehicles, forcing them to stop and then doused them in petrol to set them on fire in the southwestern province of Balochistan.

"Five NATO trucks were carrying NATO equipment back. Gunmen first fired on the first vehicle and then sprinkled petrol on all of them," Mr Iftikhar Bugti, a senior government official told AFP by telephone.

The incident happened in Bolan district, around 120km southeast of Quetta, the provincial capital.

"All five trucks have been almost completely destroyed," Mr Bugti said. One driver was slightly injured in the attack, he added.

Mr Syed Waheed Shah, another government official in Bolan, confirmed the attack.

Military officials said on Sunday that it would cost the United States US$5-6 billion (US$6.2-7.4 million)) to remove military hardware and vehicles from Afghanistan, where NATO is ending its combat mission by the end of next year.

Brigadier General Steven Shapiro described it as "one of the most challenging military transportation operations in history in terms of scale and complexity".

Most of the hardware will be flown out of landlocked Afghanistan or taken by road to Pakistan's port of Karachi, despite complications with the route.

Pakistan in July 2012 temporarily stopped NATO traffic after gunmen attacked NATO trucks, killing a driver, in the northwestern border town of Jamrud.

Islamabad also imposed a seven-month blockade on overland NATO traffic after US air strikes killed 24 Pakistani soldiers in November 2011.