Pakistan's top military and civilian leadership ordered the U.S. to leave a key airbase in the country Sunday after a deadly NATO airstrike Saturday. Secured in a secret agreement following 9/11, Shamsi airbase has been an important part of U.S. drone activity. American forces have 15 days to leave the base.

Early Saturday morning, International Security Assistance Force helicopters shelled Pakistani checkpoints, purportedly on accident, killing 24 soldiers and injuring 13. According to The Guardian, this latest incident may be something of a last straw in the strained Islamabad-Washington relationship.

The Guardian's Saeed Shah reported from Karachi:

"These attacks, which constituted breach of sovereignty, were violative of international law and had gravely dented the fundamental basis of Pakistan's co-operation with Nato/Isaf against militancy and terror," said a statement issued by the committee, which is chaired by the prime minister and includes the army chief. "Nato/Isaf attacks were also violative of their mandate, which was confined to Afghanistan."

The deaths of the Pakistani soldiers will fuel anti-American sentiment in Pakistan, a key US ally. Although there have been previous deaths of Pakistani troops caused by mistaken fire from coalition aircraft, the scale of the bloodshed this time was far greater. Pakistan's army chief, Ashfaq Kayani, put the death toll at 24. Other reports put the number killed as high as 28.

When summoned to Pakistan's foreign ministry after the attack, Ambassador Cameron Munter said the U.S. would investigate the incident, but did not admit American forces had caused the deaths Saturday.

Pakistan has attempted to kick out the U.S. in the past, but their threats have been empty. A steady deterioration in relations may mean this most recent demand will be carried out after all.

Meanwhile, NATO forces in Afghanistan anticipated retaliation for the attack after Pakistani officers made it clear they were no longer welcome. The Guardian reported elsewhere that Islamabad cut supplies to the International Security Assistance Force.

One Isaf source voiced concern that the Pakistani intelligence agency, the ISI, could go much further and use its suspected influence over insurgent groups in the tribal areas along the Afghan border to launch reprisal attacks on Nato. "This will come back at us, and at a time and a place of their [the ISI's] choosing," the source predicted. In September the chairman of the US joint chiefs of staff, Admiral Mike Mullen, said the ISI was using insurgent groups such as the Haqqani network to wage a "proxy war" in Afghanistan.

U.S. General John Allen, who has been in charge of coalition troops in Afghanistan, did not offer details of the attack or its aftermath. According to the general, though, the forecast for relations with the country is murky at best. "We don't know where all of this will end up with Pakistan," he said in an interview Sunday. "We have been good friends with them for a long, long time, and this is a tragedy."

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