Afghan security forces shot and killed one American and two British troops in two separate incidents, the latest in a rising number of attacks in which Afghan forces have turned their weapons on their foreign partners.

Monday's killings reflect a spike in tensions between Afghan and international forces that follow an American soldier's alleged massacre of Afghan civilians, the burning of Muslim holy books at a U.S. base, and uncertainty about Afghanistan's fate as foreign troops prepare to pull out AP reported.

They also come at a time when 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 that partnership is key to the U.S.-led coalition's strategy to withdraw most foreign combat forces by the end of 2014.

U.S. Marine Gen. John Allen, the top commander of U.S. and NATO forces in Afghanistan, told reporters at the Pentagon 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.

Since 2007, an estimated 80 NATO service members were killed by Afghan security forces, according to an Associated Press tally, which is based on Pentagon figures released in February. More than 75 percent of the attacks have occurred in the past two years.

Sixteen NATO service members — 18 percent of the 84 foreign troops killed so far this year — have been shot and killed by Afghan soldiers and policemen or militants disguised in their uniforms, according to the AP tally.

In one incident Monday, two British service members were killed by an Afghan soldier in front of the main gate of a joint civilian-military base in southern Afghanistan, the coalition said. Another coalition service member was shot and killed at a checkpoint in Paktika province 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, local officials said. A U.S. official confirmed that the service member killed was an American.

Maj. Ian Lawrence, a British military spokesman for Task Force Helmand, said one of the British troops was a Royal Marine and the other was a soldier from the British Adjutant General's Corps. They were killed in front of the base in Lashkar Gah, the provincial capital of Helmand province.

The soldier responsible, who had been in the Afghan National Army for four years, arrived at the gate in an army vehicle, said Ghulam Farooq Parwani, deputy commander of the Afghan army in Helmand. He was able to get close to 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, Parwani added.

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

It is not the first time that Afghan security forces have killed their British counterparts. On Nov. 3, 2009, a rogue Afghan policeman killed five British soldiers who had been advising Afghan police at a checkpoint in Helmand province.

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.

However, Wahid Muzhda, a former Taliban foreign ministry official and an analyst on issues related to the group, said the Taliban were not behind most of the latest killings.

"All these killings are not linked to the Taliban," Muzhda said. "The recent Quran burnings and the shooting spree — the killing of children— are affecting the minds of the Afghan soldiers. They think the foreigners are looking out for their own interests. They think if the foreigners are coming here to defend Afghanistan, why are they killing children?"

The trust between the Afghan forces and their international mentors is being undermined, he said.

"How is the mentor supposed to teach if he is afraid of the Afghan soldiers? They have weapons. How can he relax?"