YAKKAGHUND, Pakistan - NATO helicopters and fighter jets attacked two military outposts in northwest Pakistan on Saturday, killing as many as 28 troops and plunging US-Pakistan relations deeper into crisis.Pakistan shut down NATO supply routes into Afghanistan - used for sending in nearly half of the alliance's land shipments - in retaliation for the worst such incident since Islamabad uneasily allied itself with Washington following the Sept. 11, 2001 attacks on the United States.Islamabad also said it had ordered the United States to vacate a drone base in the country, but a senior US official said Washington had received no such request and noted that Pakistan had made similar eviction threats in the past, without following through.NATO and US officials expressed regret about the deaths of the Pakistani soldiers, indicating the attack may have been an error; but the exact circumstances remained unclear."Senior US civilian and military officials have been in touch with their Pakistani counterparts from Islamabad, Kabul and Washington to express our condolences, our desire to work together to determine what took place, and our commitment to the US-Pakistan partnership which advances our shared interests, including fighting terrorism in the region," said White House national security council spokesman Tommy Vieter.US Secretary of State Hillary Clinton and Pakistani Foreign Minister Hina Rabbani Khar spoke by telephone, as did General Martin Dempsey, the chairman of the Joint Chiefs of Staff, and Pakistani Chief of Army Staff General Ashfaq Parvez Kayani.The NATO-led force in Afghanistan confirmed that NATO aircraft had probably killed Pakistani soldiers in an area close to the Afghan-Pakistani border."Close air support was called in, in the development of the tactical situation, and it is what highly likely caused the Pakistan casualties," said General Carsten Jacobson, spokesman for the International Security Assistance Force (ISAF).He added he could not confirm the number of casualties, but ISAF was investigating. "We are aware that Pakistani soldiers perished. We don't know the size, the magnitude," he said.Pakistan's Prime Minister Yusuf Raza Gilani said the killings were "an attack on Pakistan's sovereignty", adding: "We will not let any harm come to Pakistan's sovereignty and solidarity."Pakistan's Foreign Office said it would take up the matter "in the strongest terms" with NATO and the United States, while army chief Kayani said steps would be taken to respond "to this irresponsible act"."A strong protest has been launched with NATO/ISAF in which it has been demanded that strong and urgent action be taken against those responsible for this aggression."Two military officials said up to 28 troops had been killed and 11 wounded in the attack on the outposts, about 2.5 km (1.5 miles) from the Afghan border. The Pakistani military said 24 troops were killed and 13 wounded.The attack took place around 2 a.m. (2100 GMT) in the Baizai area of Mohmand, where Pakistani troops are fighting Taliban militants. Across the border is Afghanistan's Kunar province, which has seen years of heavy fighting."Pakistani troops effectively responded immediately in self-defense to NATO/ISAF's aggression with all available weapons," the Pakistani military statement said.The commander of NATO-led forces in Afghanistan, General John R. Allen, offered his condolences to the families of Pakistani soldiers who "may have been killed or injured".Dempsey's spokesman, Colonel David Lapan, could not confirm the closure of the Pakistani border crossing to trucks carrying supplies for ISAF forces. However, he noted that "if true, we have alternate routes we can use, as we have in the past".