The insider threat to foreigners trying to mentor and strengthen Afghan security forces has existed for years but has grown more deadly.
The U.S.-led coalition routinely reports each time an American or other foreign soldier is killed by an Afghan in uniform, but the military is underreporting the number of overall attacks. The Associated Press reported earlier this month that the coalition does not report attacks in which the Afghan wounds — or misses — his U.S. or allied target. It also doesn't report the wounding of troops who were attacked alongside those who were killed.
The number of such attacks have been on the rise. So far this year there have been 19 attacks killing 12 soldiers, compared to 21 last year killing 35 coalition service members, according to NATO figures.
That compares with 11 fatal attacks and 20 deaths the previous year. In 2007 and 2008 there were a combined total of four attacks and four deaths.
U.S. officials say that in most cases the Afghans who turn their guns on their allies are motivated not by sympathy for the Taliban or on orders from insurgents, but rather act as a result of personal grievances against the coalition.
Also Sunday, a NATO service member was killed by a bomb in eastern Afghanistan, the alliance said, raising to 139 the number of foreign troops deaths so far this year.
The heads of the House and Senate intelligence committees claimed Sunday that the Taliban has actually grown stronger since 33,000 more U.S. troops were deployed to Afghanistan in 2010.
Sen. Dianne Feinstein, a Democrat, and Rep. Mike Rogers, a Republican, offered the pessimistic report on CNN's "State of the Union" after a fact-finding trip to the region where they met with President Hamid Karzai.
When asked if the Taliban's capabilities have been degraded, Feinstein said: "I think we'd both say that what we've found is that the Taliban is stronger."
More than 1,800 U.S. troops have been killed in the decade-long war. About 90,000 service members remain deployed, down from a peak of more than 100,000 last year.
Associated Press writers Robert Burns in Washington and Mirwais Khan in Kandahar contributed to this report.