Pakistan blocked a vital supply route for Nato Afghanistan forces today in retaliation for a cross-border helicopter strike by the coalition that killed three frontier troops.
The blockade appeared to be a major escalation in tensions between Pakistan and the West.
A permanent supply stoppage would put massive strains on the relationship and hurt the Afghan war effort.
Even a short halt is a reminder of the leverage Pakistan has over the United States, Britain and their allies at a crucial time in the nine-year-old war.
By late afternoon, a line of more than 150 Nato vehicles was waiting to cross the border into Afghanistan.
“We will have to see whether we are allies or enemies,” Pakistani Interior Minister Rehman Malik said.
Nato said it was investigating Pakistani reports that coalition aircraft had mistakenly attacked its forces.
Over the weekend Nato helicopters fired on targets in Pakistan at least twice, killing several suspected insurgents they had pursued over the border from Afghanistan.
Pakistan’s government protested over the attacks, which came in a month during which there have been an unprecedented number of US drone missile strikes in the north-west, inflaming already pervasive anti-American sentiment among Pakistanis.
The surge in attacks and apparent increased willingness by Nato to attack targets on the border, or just inside Pakistan, could be a sign the coalition is losing patience with Pakistan, which has long been accused of harbouring militants in its lawless tribal regions.
The border between Pakistan and Afghanistan is unmarked. Border troops wear uniforms that resemble the traditional Pakistani dress of a long shirt and baggy trousers, which could make it hard to distinguish them from ordinary citizens or insurgents.
Western officials have complained that Pakistani security forces do little to stop the movement of militants seeking to cross over into Afghanistan and attack foreign troops there.
Hours after the latest incident Pakistani authorities were ordered to stop Nato supply trucks from crossing into Afghanistan at the Torkham border post, a major entryway for Nato materials at the edge of the Khyber tribal region.
The other main route into Afghanistan in south-eastern Pakistan had received no orders to stop Nato trucks from crossing.
Some 80% of non-lethal supplies for foreign forces fighting in landlocked Afghanistan are transported over Pakistani soil after being unloaded at docks in Karachi, a port city in the south.
While Nato and the United States have alternative supply routes, the Pakistani ones are the cheapest and most convenient.