Gunmen in Pakistan have set fire to up to 40 supply trucks for NATO troops in two raids, the latest in a series of assaults on the logistical backbone of the war in Afghanistan.

The attacks were launched on the same day the United States apologized to Pakistan for a Nato cross-border incursion in which US helicopters killed two Pakistani soldiers.

A US embassy statement said a joint investigation showed U.S. helicopters had mistaken the soldiers for insurgents they had been pursuing.

The September 30 helicopter strike was the most serious of recent cross-border incidents involving NATO-led forces fighting in Afghanistan, which have stoked tensions with neighboring Pakistan.

Gunmen torched 20 fuel tankers in the first attack on Wednesday near the southwestern city of Quetta. The second attack was in the northwest.

A top administration official Ali Anan Qamar in the northwestern city of Nowshera told Reuters by telephone there were 15 to 20 tankers on fire, and that he did not know about casualties.

Pakistan's Express TV and Geo TV showed pictures of burning vehicles. Express and Dunya TV reported that 35 oil tankers had been destroyed.

In the earlier attack near Quetta, gunmen opened fire on the trucks and torched them, killing a driver.