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Afghan security forces killed three foreign troops, including two British soldiers, on Monday -- the latest in a growing number of attacks in which Afghan forces have turned their guns on their international partners.

The top commander of U.S. and NATO forces in Afghanistan said such attacks were to be expected when fighting insurgencies.

The two British service members were gunned down by an Afghan soldier in front of the main gate of a joint civilian-military base in southern Afghanistan, according to the U.S.-led coalition. Another NATO service member, whose nationality was disclosed, was shot dead at a checkpoint in eastern Afghanistan by a man who was believed to be a member of a village-level fighting force the U.S. is fostering in hopes of countering the Taliban insurgency.

International forces have faced a series of so-called "green on blue" attacks in which Afghan security forces have gunned down their international colleagues or mentors. Such strikes have become increasingly common over the past year, particularly since the burning of Korans at a U.S. base in February.

Allegations that a U.S. soldier killed 17 Afghan civilians in a shooting spree earlier this month also has stoked sentiment against foreign forces.

U.S. Marine Gen. John Allen told reporters at the Pentagon on Monday that these types of attacks are characteristic of any warfare involving insurgents.

"We experienced these in Iraq. We experienced them in Vietnam," Allen said. "On any occasion where you're dealing with an insurgency and where you're also growing an indigenous force ... the enemy's going to do all that they can to disrupt both the counterinsurgency operations" and the developing nation's security forces.

Sixteen NATO service members, including eight Americans, have been killed by Afghan security officials or militants disguised in their uniforms so far this year. That would raise to 80 the estimated number of NATO service members killed by Afghan security forces since 2007, according to an Associated Press tally based on Pentagon figures released in February. More than 75 percent of the attacks have occurred in the past two years.

British Defense Secretary Philip Hammond confirmed that two British service members had been killed earlier Monday at the main NATO base in Lashkar Gah in Helmand province.

"Details of the incident are still emerging, but it appears that a member of the Afghan National Army opened fire at the entrance gate to the British headquarters in Lashkar Gah," Hammond told the Commons. "Our thoughts, as ever, are with their families, for whom this will be a deeply personal tragedy."

Ghulam Farooq Parwani, deputy commander of the Afghan National Army in Helmand, said the shooter was from the eastern Nangarhar province and had been in the army for four years. The Afghan soldier arrived at the gate of the base in an army vehicle. He was able to approach the British troops by claiming that he had been assigned to provide security for a delegation of government officials from Kabul who were visiting the base Monday, according to Parwani.

"He got close to the foreign troops -- three or four meters (yards) -- and he opened fire," Parwani said. "Then the foreign troops killed him."

Taliban spokesman Qari Yousef Ahmadi said the shooter was an Afghan soldier who was in close contact with insurgents and had notified the Taliban of his planned attack before carrying it out.

Few details were disclosed about the second attack, which was being investigated by Afghan and coalition officials.

The coalition said only that a NATO service member was killed while approaching a checkpoint run by the Afghan Local Police. That's a force comprised of units of 250 to 300 men approved by community elders, the Afghan government and NATO forces, and overseen by the Afghan National Police. Once vetted and trained, they are given guns, uniforms and a small salary to watch over their community, similar to the Sons of Iraq program that helped turn the tide of violence in that wartorn country.

Monday's attacks come two weeks after a U.S. soldier allegedly went on a pre-dawn shooting rampage in neighboring Kandahar province, killing 17 people and wounding six. Army Staff Sgt. Robert Bales, who has been charged with 17 counts of premeditated murder, is accused of walking away from the base on possibly two occasions on the same night, and gunning down men, women and nine children while they slept in their beds.

The NATO deaths further raise tensions at a critical time, as international troops have stepped up training and mentoring of Afghan soldiers, police and government workers so that Afghans can take the lead and the foreign forces can go home.

The success of the partnership, which is the focus of the U.S.-led coalition's exit strategy, is threatened by the rising number of Afghan police and soldiers -- or militants disguised in their uniforms -- who are turning their guns on their foreign allies.

Six American troops were killed in what were believed to be revenge attacks for the burning of the Korans, although it is impossible to know the exact motive because most the shooters were killed in the incidents.

The U.S. apologized for the burning, saying the Islamic texts were mistakenly sent to a garbage burn pit Feb. 20 at Bagram Air Field, north of Kabul. But the incident raised what had been simmering animosity toward outsiders to a full boil. Deadly protests raged around the nation for six days -- the most visible example of a deep-seated resentment bred by what Afghans view is a general lack of respect for their culture and religion.