The incident took place near the town of Mithri, some 230 kilometres (140 miles) southeast of Quetta, the capital of the restive oil and gas-rich Baluchistan province which borders Iran and Afghanistan.

"Unknown gunmen fired at five NATO oil tankers. Two of them caught fire while oil leaked from three others," district administration chief Muhammad Azam told AFP.

"There were no injuries to drivers," he said.

Local intelligence officials confirmed the attack.

NATO trucks and oil tankers are regularly the targets of arson attacks blamed on insurgents attempting to disrupt two key supply lines that cross western Pakistan bound for foreign troops fighting in neighbouring Afghanistan.

Most supplies and equipment required by coalition troops in Afghanistan are shipped through Pakistan, although US troops increasingly use alternative routes through central Asia.

Baluchistan is torn by Islamist militancy, sectarian violence between majority Sunnis and minority Shiite Muslims, and a separatist insurgency by rebels seeking political autonomy and a greater share of profits from natural resources.