ISLAMABAD - Pakistan is planning to tax supplies for U.S.-led coalition troops that are shipped through its territory to landlocked Afghanistan, officials revealed, in retaliation for the recent deaths of its soldiers in a "friendly fire" incident on the border.

Under the proposal being worked up inside the government, which is set to become policy, a transit tax or fee would be imposed on every shipping container sent through Pakistan, according to senior military and civilian officials.

The move will add tens or perhaps hundreds of millions of dollars a year to the cost of the conflict, which is now a decade old. It follows the deaths of 24 Pakistani soldiers at a checkpoint that was shelled by a U.S. helicopter, the latest incident this year to roil the relationship between the antiterrorism allies.

It is thought that the government would levy about $1,500 per shipping container sent through Pakistan, along with separate charges for each fuel tanker en route to Afghanistan. That is equivalent to adding one-third to the costs of transporting a container through Pakistan.

The international coalition in Afghanistan has benefited from the free transit of goods through Pakistan for nearly a decade, under agreements forged with Islamabad. Although the coalition has reduced its logistical dependence on Pakistan in the last few years, about a third of its supplies are still trucked through the country, which is the cheapest route to Afghanistan.

The free transit of U.S. and NATO military supplies was allowed under two agreements signed in 2002.

"Under the agreement, NATO got to use our transport infrastructure for no cost, but what we got in return, we don't know. It is high time to revisit the agreement," said a senior Pakistani official, who spoke on the condition of anonymity as the new policy hasn't been announced.

The official added that the details of how the tax would work were still being formulated.

The United States has provided more than $21 billion in civilian and military aid to Pakistan since 2001.

Prime Minister Yousuf Raza Gilani said Wednesday that tougher rules of engagement were being worked out with NATO, "with the aim of providing more respect to our sovereignty."

Islamabad suspended the movement of coalition supplies altogether after the soldiers' deaths in November. The tax on NATO supplies would be the most hard-hitting of the punitive measures Islamabad imposed, but it would at least provide Pakistan with a face-saving way of reopening the route. After the deaths at the border, Pakistan also terminated the American use of an air base, Shamsi, and boycotted an international conference on Afghanistan.

Anger is intense within the Pakistani military over the deaths, which it believes were a deliberate attempt to see how far the country could be pushed. The public is demanding an end to all cooperation with the U.S.-led coalition.

It's estimated that 4,000 containers a month carry supplies to NATO troops from all three routes, meaning that about 1,300 of them would pass through Pakistan. About 1,000 fuel tankers go from Pakistan to Afghanistan each month.

The supplies to NATO's 150,000 troops are shipped to Karachi and then trucked across the country, entering Afghanistan either in its south, through the Chaman border crossing, or via the Khyber Pass to eastern Afghanistan. A much longer, more expensive supply route that enters northern Afghanistan has been developed through Europe and central Asia, while the rest of the goods are flown in, the most costly option.