QUETTA, Pakistan (Reuters) – Gunmen in southwestern Pakistan  set fire to nearly 30 tankers carrying fuel for NATO troops in Afghanistan on Saturday, an official said, two days after the United States apologized to Pakistan for a cross-border air raid that killed two Pakistani soldiers.

Suspected Islamist militants have stepped up attacks on convoys carrying supplies for NATO forces since the air September 30 air strike in northwestern Pakistan described by the U.S. ambassador to Pakistan as a terrible accident.

About 20 gunmen set fire to around 30 tankers parked outside at a roadside restaurant near the southwestern town of Sibi in a pre-dawn attack, the official said.

The tankers were on their way to the border town of Chaman.

"The attackers first fired shots and then fired small rockets at the tankers. Twenty-eight to 29 tankers caught fire," local government official Naeem Sherwani told Reuters. He said one of the paramilitary soldiers escorting the convoy was wounded.

The U.S.-backed Pakistani government is battling Taliban insurgents who remain effective despite military crackdowns on their strongholds in the northwest near the Afghan border.

Two suspected suicide bombers struck at a crowded Sufi Muslim shrine in the Pakistani city of Karachi on Thursday, killing at least seven people and wounding 65.

The U.S. apology for the September 30 cross-border raid had raised the hopes that Pakistan would reopen a vital supply route in the northwest for coalition forces which Islamabad shut after the NATO strike, citing security reasons.

A second supply route passing through southwestern Pakistan has remained open.

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