Updated

The U.S. is paying six times as much to send war supplies to troops in Afghanistan through alternate routes after Pakistan's punitive decision in November to close border crossings to NATO convoys, the Associated Press has learned.

Islamabad shut down two key Pakistan border crossings after a U.S. airstrike killed two dozen Pakistani soldiers in late November, and it is unclear when the crossings might reopen.

Pentagon figures provided to the AP show it is now costing about $104 million per month to send the supplies through a longer northern route. That is $87 million more per month than when the cargo moved through Pakistan.

While U.S. officials have acknowledged that using alternate transportation routes for Afghan war supplies is more expensive and takes longer, the total costs had not been revealed until now. The Pentagon provided the cost figures to the AP on Thursday.

U.S. officials said Thursday the elevated costs are likely to continue for some time, as U.S.-Pakistan tensions remain high and Pakistan has not yet offered to restore the transport arrangement or to begin negotiations on the matter. Until the closure, the U.S. had relied on Pakistani routes to move about one-third of all war supplies for Afghanistan.

The U.S. has given Pakistan more than $20 billion in aid since 9/11, including civilian and military assistance. But over the past year, relations with Islamabad have been strained by a series of incidents, including the U.S. assault in Pakistan last May that killed Osama bin Laden.

Pakistani leaders have also complained about repeated U.S. drone strikes into their country. The strikes, largely by the CIA, target militants hiding along Pakistan's border who launch attacks against NATO troops in Afghanistan.

The final straw, however, was the Nov. 26 cross-border attack, which hit two Pakistani border posts, enraging the Pakistani government and further eroding already shaky relations.

The U.S. blamed the errant airstrikes on a series of communications and coordination errors on both sides. American officials expressed regret but have not apologized for the incident, insisting that Pakistan fired first. Pakistan denies that and has called it an unprovoked attack.

In addition to closing the border crossings, Pakistan ordered the U.S. to vacate Shamsi air base, which the U.S. was using to launch drone strikes at al-Qaida and Taliban militants.

Over the past year or so, the U.S. military has been shrinking its reliance on the Pakistani routes, which are used to transport fuel and other non-lethal supplies. U.S. officials say they could manage indefinitely without that access if Pakistan either makes the closure permanent or offers to reopen it under unacceptable conditions.

Officials said that moves by Pakistan to briefly close the supply routes on two previous occasions after disputes with the U.S. prompted the Pentagon to begin shifting more to the northern crossings. Officials also believe that even if Pakistan eventually opens the supply routes, that there will be additional fees charged, so the alternate routes would help avoid those extra costs.

On the other hand, sending NATO convoys through Pakistan is seen by Washington as a significant piece of the overall U.S.-Pakistani partnership. Failure to reinstate those routes would signal a more severe diplomatic breach between the two countries at a critical time in the Afghan war and the ongoing battle against insurgents who seek sanctuary on the Pakistan side of the border.

According to U.S. officials, 85 percent of fuel supplies for the war effort in Afghanistan are now going through the northern supply routes, along with 30 percent of the supplies that had routinely come through Pakistan.

The northern routes connect Baltic and Caspian Sea ports with Afghanistan through Russia and Central Asia and the Caucasus. And they combine sea, rail and truck transport.

There may be, however, some movement by Pakistan to allow certain civilian Afghan supplies through the closed routes.

Dependent on Pakistan for its imports, landlocked Afghanistan has asked authorities in Pakistan to release hundreds of vehicles stacked with goods and fuel that are being held up along with NATO supplies.

Pakistani officials say they are sorting through the thousands of stranded vehicles to push through supplies for Afghans. So far, the Pakistanis have given no indication of when they will open the border for NATO supplies to Afghanistan.

There has been limited contact between top U.S. and Pakistani officials.

Last week, Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, talked by phone with his Pakistani counterpart, Army Gen. Ashfaq Pervez Kayani, their first contact since Dec. 21. Defense Secretary Leon Panetta has not spoken to Pakistani leaders since the incident.

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Associated Press Writers Bradley Klapper in Washington and Kathy Gannon in Islamabad contributed to this report.