Pakistani officials say suspected militants in the country's south have set fire to more than two dozen tankers carrying fuel for foreign troops in Afghanistan.
The attack took place one day after Pakistan shut down the Torkham border crossing in the northwest -- a vital supply route for NATO forces -- following a NATO cross-border strike that Pakistan says killed three soldiers.
Officials say a group of gunmen opened fire Friday, forcing people to flee before setting 27 tankers on fire in the southern town of Shikarpur. The vehicles were on their way to Afghanistan from the southern port city of Karachi. Police later arrested around 10 suspects.
Later on Friday, officials say assailants torched a tanker in southwestern Baluchistan province, killing two people.
The bulk of fuel and other non-lethal supplies for NATO troops in Afghanistan moves through Pakistan.
On Friday, the supply route in Pakistan's Khyber district remained closed.
The U.S. special representative to Pakistan and Afghanistan, Richard Holbrooke, Friday expressed optimism that Pakistan will quickly reopen the Torkham border crossing, saying an extended closure would have a "colossal" impact on the region's economy. Holbrooke told a conference in Washington that it was unlikely the crossing was shut entirely.
Authorities shut the border crossing Thursday, after Pakistan's military said NATO helicopters attacked a paramilitary checkpoint in the northwestern Kurram region, killing three soldiers.
It was the fourth cross-border raid in recent days.
Pakistan's ambassador to Belgium lodged a formal protest against the border incursions with NATO leaders on Friday.
And Pakistani Prime Minister Yousuf Raza Gilani told Parliament Friday that Pakistan will not tolerate an attack on Pakistani soil. He says the country will "consider other options" if its sovereignty is violated.
NATO said its helicopters entered Pakistani airspace on Thursday and hit a target only after receiving ground fire. The alliance expressed condolences to the families of the Pakistani soldiers and said it would investigate the incident.
Some information for this report was provided by AFP, AP and Reuters.