Image caption Many tankers bound for Afghanistan are now stranded in the port of Karachi

Pakistan is considering charging millions of dollars in annual taxes on Nato trucks and fuel tankers, officials have told the BBC.

The vehicles pass through Pakistan on their way to Afghanistan.

The charges might include taxes on fuel in addition to port and storage fees, they said.

The supply route is a lifeline for Nato troops but Pakistan closed it last month after 24 of its soldiers were killed in a Nato air strike.

Thousands of tankers are now stranded.

On Sunday, Prime Minister Yousuf Raza Gilani told the BBC that Pakistan may continue its blocking of Nato convoys into Afghanistan for several weeks.

On the same day, gunmen attacked tankers stranded in the province of Balochistan for the second time in four days.

The attackers shot dead a driver and destroyed seven tankers.

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Pakistan stopped the convoys in protest at US air strikes that killed the 24 troops at two checkpoints on the Afghan border.

Mr Gilani refused to rule out closing Pakistan's airspace to the US.

Credibility gap

The air strikes on 26 November marked a new low point in relations between Washington and Islamabad, which have long been strained by the US-led military campaign against militants in Afghanistan.

In a wide-ranging interview with the BBC, Mr Gilani said Pakistan and the US needed to trust each other more.

"Yes there is a credibility gap, we are working together and still we don't trust each other," he said.

"I think we have to improve our relationship so that... we should have more confidence in each other."

Nato forces in Afghanistan rely significantly on overland supply routes from the Pakistani sea port of Karachi, which enter Afghanistan through the Khyber Pass.

Hundreds of lorries have been camped out next to border crossings, waiting for the crisis to blow over.

Nato has apologised for the air strikes, calling them a "tragic unintended incident".