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ISLAMABAD, Pakistan - Assailants in Pakistan launched two separate attacks Friday on vehicles carrying fuel for NATO and American forces in Afghanistan, highlighting the vulnerability of the U.S.-led mission a day after Pakistan closed a major border crossing.

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.

Earlier Friday, suspected militants torched 27 tankers carrying oil for troops in Afghanistan in Sindh province.

Around 80 per cent of the fuel, spare parts, clothing and other non-lethal 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.

The Pakistani government shut the Torkham border crossing in the northwest on Thursday in apparent protest of a NATO helicopter incursion that killed three of its soldiers on the border. It kept open the Chaman crossing in Baluchistan, where it seemed likely the vehicles attacked Friday were heading.

The closure raised tensions between Pakistan and the United States, which have a close but often troubled alliance in the fight against militants.

Islamist militants regularly attack NATO supply tankers in Pakistan, mostly in the northwestern border region where their influence is stronger.

In Sindh, around 10 gunmen attacked the tankers 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.

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

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. Some of the attacks are believed to be the work of criminals or in Baluchistan, separatists. Some officials allege truck owners may be behind some of them, perhaps to fraudulently claim insurance.

The vast majority of the convoys, however, through the country unharmed and the frequency of attacks reported in the media does not appear to have risen much, if at all, over the last two years.

In recent years, the alliance has sought to shift more of the supplies through Central Asian countries north of Afghanistan and Russia, aware of the problems of relying too much on Pakistan, which some argue does not share America's strategic goals in the region.

There is a risk, albeit small, that militant attacks could one day seriously squeeze supplies. But the overriding concern is that hosting the supply routes gives Islamabad immense leverage in its relationship with Washington. The United States cannot force Pakistan to, say, crack down on militants in the northwest behind attacks in Afghanistan because Islamabad holds a trump card: it can cut off most of the supplies to the war whenever it wants.

Pakistani security forces provide guards for the trucks and tankers in the northwest, but generally do not do so in southern 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.

In Brussels on Friday, 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," Defence Minister Chaudhry Ahmed Mukhtar said.

Opinion polls show many Pakistanis regard the United States as an enemy, and conspiracy theories abound of U.S. troops wanting to attack Pakistan and take over its nuclear weapons. The Pakistani government has to balance its support for the U.S. war in Afghanistan -- and its need for billions of dollars in American aid -- with maintaining support from its own population.

The decision to close to the border has underscored the uneasy relations.

Pakistan said two NATO choppers fired on one of its border posts in the northwest's Kurram tribal region, killing three Pakistani soldiers Thursday. NATO said its helicopters entered Pakistani airspace and hit a target only after receiving ground fire. The alliance expressed condolences to the families of the soldiers and said it would investigate the incident.

It was the third alleged incursion by NATO helicopters into the northwest in the last week.

A lengthy closure of Torkham would place intense strain on the U.S.-Pakistani relationship and hurt the Afghan war effort. But that is seen as unlikely, as neither Islamabad nor Washington can afford a meltdown in ties at a crucial time in the 9-year-old war.

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."