KABUL, Afghanistan — The toll in a wave of attacks against coalition forces by their Afghan counterparts rose this week, underscoring the increasing tempo of the so-called green-on-blue assaults this year.

Details have emerged about two such cases that both occurred Wednesday. In one, NATO and Afghan officials on Friday confirmed that two Afghan policemen opened fire on coalition troops in the Zhare district, near the city of Kandahar, wounding two NATO soldiers before they were themselves killed.

In the other — in which a coalition soldier was killed in Kandahar Province by a man in an Afghan soldier’s uniform — a California newspaper, The Ventura County Star, identified the victim as Andrew Britton, 25, a Green Beret whose identity and death were confirmed by a family friend in Simi Valley, Calif.

So far this year, there have been 11 attacks by Afghan security forces against coalition soldiers, resulting in the deaths of 18 people, 10 of them Americans, according to a spokesman for the NATO-led International Security Assistance Force in Afghanistan. That is 20 percent of the total of NATO combat fatalities this year. In 2011 there were 21 such attacks leading to 35 deaths, he said. Last year, a classified study commissioned by the American military found that 58 American combat fatalities, 6 percent, came in 26 green-on-blue attacks from May 2007 to May 2011.