pbr Unknown attackers have targetted tankers carrying supplies for NATO troops in Pakistan.br ppbr The attack came on the outskirts of the town of Shikarpur and is thought to be in retaliation for movements into the country by the international organisation.br ppbr It comes a day after authorities blocked a supply route for troops, angered by the killing of three soldiers in airstrikes by NATO helicopters.br ppbr A Paramilitary Rangers Force official said the attackers fired at the tankers before fleeing the scence. No one was injured.br ppbr Pakistan is a crucial ally for the United States in its efforts to stabilise Afghanistan, but analysts say border incursions and disruptions in NATO supplies underline growing tensions in the relationship.br ppbr The bulk of supplies for the troops in Afghanistan goes through Pakistan.