By Riaz Sohail
BBC News, Karachi
Nato supply convoys travelling through Pakistan to Afghanistan have regularly come under attack in the past, but following Pakistan's decision to block their route through the Khyber Pass, they now face an even bigger security threat.
Hundreds of tankers and trucks have been left stranded on highways and depots across Pakistan, with little or no security.
Taliban militants have regularly been targeting the convoys, even when they are heavily protected.
But many believe it is not just the militants who pose a security threat to the convoys.
The owners of oil tankers being used to supply fuel to Nato in Afghanistan say some of the attacks on their convoys are suspicious.
They say there is evidence to suggest that bombs have been planted in many of vehicles by the "Nato contractors" - individuals or companies who have been contracted by Nato to supply fuel and goods to forces in Afghanistan.
The contractors subsequently hire the transporters who then carry the goods.
Dost Mohammad, an oil tanker owner from Nowshera district, said a Nato contractor had recently been caught trying to plant a bomb in an oil tanker.
"This happened in the area of Paiyee, when he was putting the bomb under the vehicle."
"At that time, a few men also opened fire on the tankers. The deputy later told the police that he had been told to plant the bomb by the contractor."
Dost Mohammad said the contractor had apparently sold off the fuel first.
"Only 2,000 litres from the original 50,000 litres had been left in the tanker to cover up the crime," he said...