ISLAMABAD (Reuters) - Pakistan ratcheted up pressure on NATO on Monday over a cross-border attack that killed 24 Pakistani soldiers at the weekend, threatening to drastically reduce cooperation on peace efforts in Afghanistan.
The incident has hurt Washington's efforts both to ease a crisis in relations with Islamabad and stabilize the region as it tries to wind down the war in Afghanistan.
"This could have serious consequences in the level and extent of our cooperation," military spokesman Major General Athar Abbas told Reuters.
Pakistan has a long history of ties to militant groups in Afghanistan so it is uniquely positioned to help bring about a peace settlement, a top foreign policy and security goal for the Obama administration.
Washington believes Islamabad can play a critical role in efforts to pacify Afghanistan before all NATO combat troops pull out in 2014, so it can't afford to alienate its ally.
Adding a new element to tensions, and a diplomatic boost for Islamabad, Pakistan's ally China said it was "deeply shocked" by the incident and expressed "strong concern for the victims and profound condolences for Pakistan."
"China believes that Pakistan's independence, sovereignty and territorial integrity should be respected and the incident should be thoroughly investigated and be handled properly," Foreign Ministry spokesman Hong Lei said in a statement on the ministry's website.
Pakistan has been trying to move closer to Asian powerhouse China as ties with the United States have suffered.
China and Pakistan call each other "all-weather friends" and their close ties have been underpinned by long-standing wariness of their common neighbor, India, and a desire to hedge against U.S. influence across the region.
On Saturday, NATO helicopters and fighter jets attacked two military outposts in northwest Pakistan, killing the 24 soldiers and wounding 13 others, the army said.
Pakistan's military denied reports that NATO forces in Afghanistan had come under fire before launching the attack. Abbas said the attack lasted two hours despite warnings from Pakistani border posts.
"They were contacted through the local hotline and also there had been contacts through the director-general of military operations. But despite that, this continued," he said.
After a string of deadly incidents in the lawless and confusing border region, NATO and Pakistan set up a hotline that should allow them to communicate in case of confusion over targets, and avoid friendly fire.
NATO described the killings as a "tragic, unintended incident" and said an investigation was underway. A Western official and an Afghan security official who requested anonymity said NATO troops were responding to fire from across the border.
Pakistan's military said the strike was unprovoked and has reserved the right to retaliate.
Both explanations are possibly correct: that a retaliatory attack by NATO troops took a tragic, mistaken turn in harsh terrain where differentiating friend from foe can be difficult.
An Afghan Taliban commander, Mullah Samiullah Rahmani, said the group had not been engaged in any fighting with NATO or Afghan forces in the area when the incident took place.
But he added Taliban fighters control several Afghan villages near the border with Pakistan.
A similar cross-border incident on Sept 30, 2010, which killed two Pakistani service personnel, led to the closure of one of NATO's supply routes through Pakistan for 10 days.
The attack was the latest perceived provocation by the United States, which infuriated and embarrassed Pakistan's powerful military in May with a unilateral special forces raid that killed al Qaeda leader Osama bin Laden.
Last month, U.S. Secretary of State Hillary Clinton visited Islamabad and held a town hall meeting to try and win over Pakistanis, held talks with her counterpart and urged all sides to seek peace in Afghanistan.