Pakistani staff working on half the 10,000 trucks tasked with carrying NATO supplies overland into Afghanistan are on strike to protest against new working arrangements, their union said Thursday.

It was not immediately clear how long the strike would continue or what effect it would have on the transit of supplies into Afghanistan. An increasing share of NATO goods are also trucked into the war-torn country from the north.

Pakistan in July lifted a seven-month blockade on NATO goods travelling overland to Afghanistan, which had been introduced to protest against botched US air raids that killed 24 Pakistani troops.

It then temporarily halted supplies after gunmen attacked a convoy of NATO trucks on July 24, killing a driver, in the northwestern town of Jamrud.

"There are more than 10,000 trucks engaged in NATO supplies and more than half have gone on strike," said Hanif Khan Marwat, secretary general of the All Pakistan Goods Transporter Union.

He said that vehicles already en route to the Afghan border would stop at the crossings of Chaman and Torkham.

They are protesting against moves by the Federal Bureau of Revenue (FBR) to force truckers to work through one of 40 companies.

Officials say the changes are designed to improve security and cut down on theft, but truckers say they prevent them from also working on other assignments.

"When we tell the companies that we want our truck back they demand up to 25,000 rupees ($257) per vehicle which is not justified," said Marwat.

"We should not be bound to the new system. We will cooperate in security measures with the authorities, but we should be allowed to work as freely as before," he said.

An FBR official told AFP that the bureau was looking into the situation and said the matter would be resolved soon, without indicating how.

"We have introduced this system for better security and a check on theft, which was quite high in the previous system," said a customs official.