SHIKARPUR, Pakistan — Suspected militants in southern Pakistan set ablaze more than two dozen tankers carrying fuel for foreign troops in Afghanistan today, highlighting the vulnerability of the U.S.-led mission a day after Pakistan closed a major border crossing.

The Pakistani government shut the Torkham border in the northwest in apparent protest at a NATO helicopter incursion that killed three of its soldiers on the border. The events raised tensions between Pakistan and the United States, which have a close but often troubled alliance in the fight against militants. Pakistan also lodged a formal protest with NATO today.

The convoy of tankers attacked today was likely headed to a second crossing in southwest Pakistan that was not closed. It was not clear if the vehicles had been rerouted because of the closure at Torkham.

Around 80 percent of the fuel, spare parts, clothing and other nonlethal supplies for foreign forces in landlocked Afghanistan travels through Pakistan after arriving in the southern Arabian sea port of Karachi. The alliance has other supply routes to Afghanistan, but the Pakistani ones are the cheapest and most convenient.

Islamist militants occasionally attack NATO supply tankers in Pakistan, mostly in the northwest where their influence is stronger. Thursday's strike was in Sindh province, far from the border, and might be taken as a sign that the insurgents are expanding their reach.

Around 10 gunmen attacked the vehicles when they were parked at an ordinary truck stop on the edge of Shikarpur town shortly after midnight. They forced the drivers and other people there to flee before setting the fires, said police officer Abdul Hamid Khoso. No one was wounded or killed.

Another officer, Nisar Ahmed, said the tankers had arrived in Shikarpur from the southern port city of Karachi and were heading to Quetta, a major city in the southwest. From there, the road leads to the Chaman border crossing.

Attacks on NATO and U.S. supply convoys in Pakistan give militants a propaganda victory, but coalition officials say they do not result in shortages in Afghanistan.