Trucks carrying supplies for NATO troops in Afghanistan crossed the Pakistani border on Thursday for only the second time since Islamabad agreed to lift a seven-month blockade, officials said.

Pakistan closed overland routes for NATO convoys going to its war-torn neighbour after botched US air strikes killed 24 Pakistani soldiers in November, plunging ties between the "war on terror" allies to a new low.

Islamabad agreed to reopen the routes after US Secretary of State Hillary Clinton on July 3 apologised for the deaths, but very few vehicles have crossed the border.

Thousands of truck owners are awaiting compensation before going back to work, and drivers say the trips into Afghanistan are too dangerous and too poorly paid.

The Pakistani Taliban have threatened to attack NATO supply trucks and kill drivers if they resume trips to Afghanistan.

On Thursday, the umbrella militant organisation threatened a further wave of attacks, and claimed responsibility for shooting nine police prison officers being trained in the eastern city of Lahore.

Prime Minister Raja Pervez Ashraf ordered authorities to allow the more than 1,500 vehicles stuck in Pakistan to cross into Afghanistan following talks with US ambassador Cameron Munter, his spokesman Akram Shaheedi told AFP.

But on Thursday customs officials told AFP that only another four NATO trucks carrying food supplies had gone into Afghanistan at the Chaman border post in Pakistan's remote southwest.

In the northwestern tribal district of Khyber, officials said seven to 10 trucks loaded with NATO supplies were due to cross the Torkham post for the first time since Pakistan agreed to resume supplies.

Later in the day Mohammad Fayyaz, an administrative official at Torkham, told AFP that four trucks loaded with food had crossed the frontier into Afghanistan.

But in Karachi, where NATO containers begin their long journey to Pakistan's two Afghan border crossings from the Arabian Sea port, many are waiting for compensation from subcontractors for being out of work for seven months.

"We are too wary, too anxious and too cautious about the situation. It was dangerous to go overland before the government ban, but now the dangers have increased," Akram Khan Durrani, president of the All Pakistan Oil Tankers Owners Association, told AFP.

"No one from the authorities have contacted us properly and assured us of foolproof security," he said.

Rana Mohammad Aslam, vice president of the All Pakistan Goods Carrier Association, said NATO subcontractors were supposed to pay $6,000 compensation per vehicle to truck owners.

"Except for some trucks which were stuck elsewhere and have settled their payment issues with the contractors, none have started moving," he told AFP from Karachi.

"Subcontractors have started installing satellite trackers on trucks as a means of security, but still there is no nod from the government, which has to arrange foolproof security for the operation."

Officials in customs and at the ports and shipping ministry, who wished not to be named, said it would still "take a few days" to finalise compensation and were unable to give a specific date for trucks to leave Karachi.

The interior ministry in southern Sindh province, of which Karachi is capital, said it was finalising a security plan, but declined to go into details.

Tensions have been high among right-wing and extremist organisations since Pakistan last week decided to reopen its Afghan border to NATO supply convoys, ending a seven-month blockade following negotiations with US officials.

The Defence Council of Pakistan, a coalition of right-wing and hardline Islamist groups opposed to the country's alliance with Washington, has led protests against the resumption of supplies for NATO troops fighting the Taliban in Afghanistan.

"You see how many people are opposing it? Earlier, we only feared the Taliban, but now we're afraid of many people and groups," said truck owner Mohammad Asghar.