A gunman killed a driver and torched more than a dozen tankers carrying fuel to NATO troops in Afghanistan on Wednesday.

It was the sixth attack on convoys taking supplies to Afghanistan since Pakistan closed a key border crossing almost a week ago.

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The closure has left hundreds of supply trucks stranded along highways and traffic backed up to the one remaining route into Afghanistan.

"There were about nine to twelve people, who have come here and carried out this sabotage activity," said Mir Wise, Pakistani senior superintendent of police. "They were armed and they have fired shots, and then they put some of the tankers on fire, about 13 of them caught fire."

The Pakistani government shut down the Torkham crossing along the fabled Khyber Pass last Thursday after a NATO helicopter attack in the border area killed three Pakistani troops. A Pentagon spokesman said an investigation of the incident should be finished on Wednesday.