ISLAMABAD, Pakistan (AP) — Assailants launched two separate attacks on tankers carrying fuel for foreign troops in Afghanistan on Friday, showing the vulnerability of NATO supply lines a day after the Pakistani government itself shut one down.

The events stand to complicate a difficult war in Afghanistan, especially if the Torkham border crossing along the fabled Khyber Pass remains closed for long. They are a reminder of the leverage Pakistan has over the United States just as Washington seeks the help of its uncomfortable ally at a crucial point in the 9-year-long conflict.

They also highlight the importance of recently opened supply routes into landlocked Afghanistan through central Asian states to its north. Those routes are safer, but the Pakistani lines from the Arabian seaport of Karachi north to Kabul and Kandahar in Afghanistan are cheaper and account for most of NATO’s non-lethal supplies.

Pakistan shut down the Torkham border crossing — the most important NATO supply into Afghanistan — on Thursday in apparent protest of a NATO helicopter attack that killed three Pakistani soldiers on the frontier. It was the third such incursion into Pakistan in less than a week.

The other NATO supply line through Pakistan remained open — the Chaman crossing in Baluchistan, where it seemed likely the tankers were heading.

A lengthy closure of Torkham would place intense strain on the U.S.-Pakistani relationship and hurt the Afghan war effort. But a long shutdown continued to be seen as unlikely.

Senior U.S. officials acknowledged high tension between the two capitals that crested with the border closure.

On the Pakistani side, the incursions into Pakistan by U.S. forces fighting in Afghanistan provoked an unusually strong government condemnation. On the U.S. side, publication of a video that may show Pakistani military officers summarily executing insurgents threatened to undermine public and congressional support for U.S. aid.

Keeping the crossing closed briefly could also allow hot tempers to cool, and provide Pakistan’s fragile civilian government a visible symbol that it is willing to stand up to its U.S. backers.

Pakistani officials gave mixed signals Friday.

In Brussels, Pakistani Ambassador Jalil Abbas Jilani met with NATO leaders and lodged a formal protest over the border incursions. In Pakistan, government officials said they had to take a stand.

“If the NATO forces keep on entering into Pakistan and carrying out attacks, then (the) only option we have — we should stop the movement of the containers,” Defense Minister Chaudhry Ahmed Mukhtar said.

But the Pentagon said Pakistani officials with whom the U.S. military had been in touch rejected the idea that the closure was retaliation for the border incursion and killings, saying instead that it was done to tighten security amid tension in the region arising from the incidents.

“What the Pakistani military described to us is that the closure of the gate was due to their concerns over rising tensions — it was to them a security issue,” said Marine Corps Col. Dave Lapan, a Defense Department spokesman. “And as you see, overnight last night some individuals attacked a group of tanker trucks.”

Richard Holbrooke, the Obama administration’s special representative for Pakistan and Afghanistan, said he was confident the crossing will reopen soon. He argued that it would make no economic sense for Pakistan to continue to restrict the flow of materials to U.S.-led NATO forces.

“Once they start closing that thing it’s going to have a colossal effect on the region,” Holbrooke told a conference in Washington.

It was unclear whether there was any link between the two attacks Friday and the border closure.

Militant attacks on NATO convoys are quite common, but extremists in Pakistan are media-savvy and could be seeking to exploit an uncomfortable moment for NATO by stepping up their campaign.

Islamist politicians and groups protested the NATO helicopter incursion and a recent increase in CIA drone strikes against al-Qaida and Taliban targets in Pakistan’s border region. They relished the opportunity to portray America — not the militants — as the enemy.

“Oh Pakistani soldiers, shoot down the drones, cut the NATO supplies and abandon American’s war,” said a statement from the Pakistani branch of the international Islamist group, Hizb ut-Tahrir.

Just after midnight, about 10 suspected militants attacked 27 tankers parked at an ordinary truck stop on the edge of Shikarpur town in Sindh province, far from the Afghan border. They forced the drivers to flee by firing in the air before setting them ablaze, said police officer Abdul Hamid Khoso.

A truck driver and his assistant were burned alive in the second attack on a single tanker in the parking lot of a restaurant in southeastern Baluchistan province, said police officer Mohammad Azam. He said “anti-state elements” were behind the attack. That term could refer to Islamist militants or separatist rebels active in the region.

Most of the attacks on the convoys are in the northwest, where militant influence is stronger.

Pakistani security forces provide guards for the trucks and tankers in the northwest, but generally do not do so in south and central Pakistan, where attacks are rarer. Pakistani security officials had warned after two alleged NATO helicopter incursions last weekend that they would stop providing protection to NATO convoys if it happened again.

Attacks on convoys in Pakistan give militants a propaganda victory, but coalition officials say they do not result in shortages in Afghanistan. Some attacks are believed to be the work of criminals, with officials alleging truck owners may be behind some of them, perhaps to claim insurance fraudulently.

At Torkham, some 150 containers were waiting Friday for the border to reopen. The truck drivers were getting impatient and worried about the prospect of militant attacks.

“I might have not come here with NATO material if I knew that I will have to face this problem,” said Shalif Khan. “We are forced to spend the day and the night in the open. We do not have any security here.”

___

Associated Press writers Riaz Khan in Torkham, Pakistan, and Robert Burns and Pauline Jelinek in Washington contributed to this report.