NATO helicopters based in Afghanistan carried out at least two airstrikes in Pakistan that killed more than 50 militants after the insurgents attacked a small Afghan security outpost near the border, officials said Monday. 
 
 NATO justified the strikes based on "the right of self-defense." 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 into Pakistani airspace if they are in hot pursuit of a target. 
 
 The tribal area where the strikes took place is largely controlled by militants who regularly carry out attacks against NATO troops in Afghanistan. The U.S. rarely uses manned aircraft to carry out strikes in North Waziristan and instead relies on drone attacks that American officials refuse to acknowledge publicly. 
 
 The first strike took place Saturday after insurgents based in Pakistan attacked an Afghan outpost in Khost province, 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," said Donald. "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. 
 
 'Operating in self-defense' 
 
 The second attack occurred when helicopters returned to the border area and were attacked by insurgents based in Pakistan, Donald said. 
 
 "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. 
 
 The strike killed at least four militants, Johnson said. 
 
 Citing ISAF, the BBC reported that no civilians were killed in the operation, but added that this had not been independently confirmed. 
 
 The drone flights have caused resentment in overwhelmingly Muslim Pakistan, where anti-American feeling runs high. 
 
 U.S. officials say the pilotless drones are one of the most effective weapons against militants. The strikes have killed senior Taliban and al-Qaida figures. 
 
 A new book by veteran U.S. journalist Bob Woodward claims that the Central Intelligence Agency also has a 3,000-man "covert army" of elite, well-trained Afghans, who conduct secret operations in Pakistan, according to the Washington Post. 
 
 Meanwhile, 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. 
 
 Donald, the NATO spokesman, said officials were still investigating and could not confirm or deny reports of the attack in Kurram. 
 
 The Pakistani military could not be reached to comment on the NATO attacks. ||||| (CNN) -- NATO helicopters crossed into Pakistani airspace from Afghanistan in pursuit of insurgents over the weekend, killing 49 people, a spokesman told CNN Monday. 
 
 Crossing the border did not violate the International Security Assistance Force rules of engagement, Maj. Michael Johnson said. 
 
 Pakistan is very sensitive about United States-led military operations on its territory and issued a strong protest Monday. 
 
 Pakistan called the incursions "a clear violation and breach" of the United Nations rules for foreign forces in Afghanistan. 
 
 The United Nations "mandate terminates/finishes at the Afghanistan border. There are no agreed hot pursuit rules," Pakistan's Foreign Ministry said in a statement. 
 
 ISAF spokesperson Maj. Sunset Belinsky confirmed later Monday that U.S. helicopters were involved in at least three border incidents over the weekend. 
 
 According to Belinsky, the first one on Saturday was "self-defense," with Apache helicopters pursuing insurgents across the border about a kilometer (about a half a mile) into Pakistani territory and opening fire. 
 
 Belinsky said the NATO forces communicated with the Pakistani military after the incident, in which the insurgents had opened fire on the helicopters and then crossed the border into Pakistan. 
 
 After that, a second NATO weapons team went to assess what happened in the first incident and came under fire from the Pakistani side of the border, Belinsky said. The second team returned fire, she said, adding it was unclear if the second team actually entered Pakistani air space. 
 
 Earlier, Johnson could not say whether the insurgents were killed in Afghanistan or Pakistan, or even whether the NATO helicopters fired their weapons in Pakistani territory. 
 
 The U.S. helicopeters were responding to an insurgent attack on a remote Afghan National Security Force outpost in Khost province on Friday, ISAF said in a statement. 
 
 "An air weapons team in the area observed the enemy fire, and following International Security Assistance Force rules of engagement, crossed into the area of enemy fire," the statement said. 
 
 Johnson, the ISAF spokesman, could not say how often NATO forces crossed into Pakistani airspace -- but said it had happened again Monday morning. 
 
 NATO is still getting the details on that incident, which he called "much smaller." 
 
 CNN's Richard Allen Greene, Fred Pleitgen and Nicola Hughes contributed to this report.
– NATO helicopters crossed from Afghanistan into Pakistan at least twice over the weekend, and again this morning, in pursuit of Taliban insurgents, CNN reports. At least 49 people were killed in the attacks Friday and Saturday. NATO initially denied those attacks, according to the New York Times, but now confirms them. NATO says the attacks conform with international rules of engagement, justifying them under the “right of self-defense” according to MSNBC. The attacks were unusual because they were carried out by piloted helicopters rather than unmanned drones. Pakistan is extremely sensitive about US-led operations in its territory. But hey, when you're already doing this, what's a couple of airstrikes?