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KABUL, Afghanistan - Pakistan disputed NATO's claim Monday that its forces have the right of hot pursuit across the Afghan border after coalition helicopters launched airstrikes that killed more than 50 militants who had escaped into Pakistan following an attack on an Afghan security post.

Pakistan said it had strongly protested to NATO over the airstrikes, which a coalition spokesman justified on grounds of "self-defence." Pakistan is sensitive about attacks on its territory, but U.S. officials have said they have the right to cross a few kilometres into Pakistani airspace if they are attacked and in hot pursuit of a target.

Pakistan denied Monday such an understanding exists.

Its Ministry of Foreign Affairs said in a press release the mandate of foreign troops in Afghanistan ends at the Afghan border and said the strikes were a violation of its sovereignty. Pakistan said that unless corrective measures are implemented, it will have to "consider response options."

NATO reported it launched two airstrikes on Saturday, and Pakistani intelligence officials reported a third attack on Monday -- all in tribal regions located opposite an increasingly volatile eastern region of Afghanistan. It was not clear which militant group was targeted, but the al Qaeda-linked Haqqani faction, which launches frequent attacks on NATO and Afghan forces, is particularly active there.

The first strike took place after insurgents based in Pakistan attacked the outpost in Afghanistan's Khost province, right across the border from Pakistan's North Waziristan tribal region, said U.S. Capt. Ryan Donald, a spokesman for the NATO-led International Security Assistance Force in Afghanistan.

"The ISAF helicopters did cross into Pakistan territory to engage the insurgents," Donald said. "ISAF maintains the right to self-defence, and that's why they crossed the Pakistan border."

Another ISAF spokesman, U.S. Maj. Michael Johnson, said the first airstrike killed 49 militants. NATO officials were able to assess the number of militants killed in the airstrikes by using gun cameras mounted on the helicopters, according to ISAF.

Abdul Hakim Ishaqzie, the provincial police chief in Khost, cited a higher death toll of around 60 militants. He said police at checkpoints at the border came under attack, engaged the militants in a gun battle and then called for help, prompting the helicopter strikes.

The air strikes occurred on the Pakistan side as militants fled, but police said they were able to go in and count the bodies, collecting ammunition and weapons from the battlefield.

A second attack occurred when helicopters returned to the border area and were attacked by insurgents based in Pakistan, Donald said. It killed at least four militants.

"The helicopters returned to the scene and they received direct small arms fire and, once again operating in self-defence, they engaged the insurgents," Donald said.

Pakistani intelligence officials said two NATO helicopters carried out a third strike inside Pakistani territory on Monday morning, killing five militants and wounding nine others.

The strike occurred in the village of Mata Sanger in the Kurram tribal area, which is directly across the border from the Afghan provinces of Paktia and Nangarhar, said the officials, speaking on condition of anonymity because they were not authorized to talk to the media.

NATO would not immediately confirm that.

Pakistan is a key ally of the U.S. and NATO forces fighting in Afghanistan, but Islamabad says it does not allow foreign troops to conduct combat operations in its territory and relations between the two sides have often been uneasy. Despite Pakistan's own military operations against Pakistani Taliban fighters, it has resisted pressure to move against the Haqqani faction which does not attack the Pakistani state.

However, Pakistan's strong protest Monday against the NATO airstrikes stands in contrast to the muted criticism that has accompanied a sharp rise in suspected attacks by U.S. unmanned drones in the country's tribal areas, particularly North Waziristan.

Pakistan did not comment Monday on a suspected U.S. missile strike that Pakistani intelligence officials said killed four people near Mir Ali, a major town in the North Waziristan. It was the 20th such attack this month. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

Pakistan has criticized drone attacks as violations of the country's sovereignty in the past, but that criticism has died down in recent months. It's possible that Pakistani officials no longer feel the need to protest every drone strike -- believed conducted by the CIA -- since they occur so often. Attacks by manned aircraft inside Pakistani territory, however, are quite rare. Also, U.S. officials refuse to publicly acknowledge the drone strikes, whereas NATO confirmed Saturday's helicopter attacks -- which may have prompted Pakistan to make a public response.

NATO said they didn't have a record of how many times manned aircraft have carried out strikes inside Pakistan.

In June 2008, Pakistan accused the U.S. of killing 11 of its soldiers when aircraft bombed their border post in the Mohmand tribal area. U.S. officials said coalition aircraft dropped bombs during a clash with militants. Though they expressed regret over the incident, they said the action was justified.

Meanwhile, in southern Afghanistan, NATO pressed ahead Monday with a combat operation to drive Taliban fighters from areas around the southern city of Kandahar in the insurgent heartland.

A top NATO officer said the alliance a few days ago had launched the "kinetic," or combat, phase of "Operation Dragon Strike," a joint military push with Afghan forces around Kandahar.

The push in Kandahar is seen as a part of the Obama administration's strategy to turn around the war as insurgents undermine the ability of an Afghan government to rule much of the country. Kandahar remains particularly dangerous; seven U.S. troops have been killed in Kandahar this month. Another three have been killed in the south, but no further details have been released.

Coalition forces were moving into two or three areas around Kandahar at once to pressure the Taliban "so they don't get the chance to run away," said Shah Mohammad Ahmadi, chief of Arghandab district northwest of the city. "Before, when we have tried to get rid of the Taliban, when we cleaned one area we found more Taliban in a different one."

In another volatile section of the nation, British officials said Monday that they were in contact with Afghan authorities about the disappearance of a British aid worker and three of her Afghan colleagues. The four were ambushed Sunday as they travelled in two vehicles in northeastern Kunar province. Police fought a gunbattle with the kidnappers near the attack site before the assailants fled, Kunar police chief Khalilullah Zaiyi said.

The British Embassy in Kabul said officials were working closely with local authorities and said the worker's family had been contacted.

Steven O'Connor, communications director for Development Alternatives Inc., a global consulting company based in the Washington, D.C., area, said late Sunday its employees, including a British national, were involved. The company works on projects for the U.S. Agency for International Development in Afghanistan.

NATO also said Monday they confirmed an Afghan civilian was killed by a coalition service member in the Alisheng district of Laghman province on Sunday. It said an investigation is ongoing.