KABUL, Afghanistan — NATO helicopters in eastern Afghanistan launched rare airstrikes into Pakistan, killing more than 50 militants and prompting a protest from Islamabad, while international forces pressed ahead with a combat operation to drive Taliban fighters from areas around the southern city of Kandahar in the insurgent heartland.

The airstrikes into Pakistan came after the insurgents attacked a small Afghan security outpost near the border. NATO justified the strikes based on "the right of self-defense," a spokesman said. Pakistan is sensitive about attacks on its territory, but U.S. officials have said they have an agreement that allows aircraft to cross a few miles (kilometers) into Pakistani airspace if they are in hot pursuit of a target.

Pakistan denied Monday such an agreement exists. The 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."

The first strike took place Saturday after insurgents based in Pakistan attacked the outpost in Afghanistan’s eastern province of Khost, which is located right across the border from Pakistan’s North Waziristan tribal area, 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-defense, and that’s why they crossed the Pakistan border."

The strike killed 49 militants, said U.S. Maj. Michael Johnson, another ISAF spokesman. 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 miliants 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.

"It was a very effective operation against the militants," said Ishaqzie. "Around 60 militants were killed. Police forces collected ammunition and weapons from the battlefield."

The 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-defense, 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’s strong protest of 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. Pakistani officials have 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.

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

In the south, 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."

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 intended to rid the area of insurgents and interrupt their ability to move freely and stage attacks.

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.

"It’s ongoing. It’s a clearing operation consisting mainly of U.S. troops alongside Afghan national security forces," said German Brig. Gen. Josef Blotz, a NATO spokesman. "This is one of several operations going on in the Kandahar operation. This is going according to plan."

NATO said militants have fought back with rocket-propelled grenades and small-arms fire, but Blotz said no Afghan or coalition troops have been killed.

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 traveled 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.

Also Monday, Poland’s military said a Polish soldier died of injuries sustained when a land mine exploded under his patrol vehicle in the eastern province of Ghazni. He was is the 21st Polish soldier to die in Afghanistan. Poland has some 2,600 troops in the country.

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Abbot reported from Islamabad. Associated Press Writer Rasool Dawar in Mir Ali, Pakistan, contributed to this report.