KABUL, Afghanistan - Civilian deaths caused by the war in Afghanistan have dropped for the first time in six years, the United Nations said Tuesday in its annual report on the conflict's toll on non-combatants.

The report linked the 12 percent drop in civilian deaths in 2012 to reduced ground fighting by the warring sides, chiefly the Taliban and U.S. troops; a decrease in the number of NATO coalition airstrikes; and fewer suicide attacks by the insurgents.

However, U.N. officials noted increasing threats to non-combatants last year because of the re-emergence of armed groups, including pro-government militias, especially in the country's north and northeast. Those regions generally have been regarded as secure compared with the south and east, where the Taliban-led militants are most active.

The United Nations recorded 2,754 civilian deaths in 2012, compared with 3,131 in 2011. Since 2007, when the world body began keeping these statistics, a total of 14,728 civilians have been killed in the conflict.

For years, civilian deaths caused by foreign troops have been an issue between President Hamid Karzai and his Western backers even though the U.N. says militants are responsible for far more such casualties - 81 percent in 2012 - than NATO forces.

Eight percent of civilian casualties were attributed to coalition forces, the report said; 11 percent percent of the deaths were not linked to a specific cause.

"The decrease in civilian casualties … is very much welcome," Jan Kubis, the U.N. special envoy for Afghanistan, said in the report. "Yet, the human cost of the conflict remains unacceptable."

The use of roadside bombs by insurgents was the "single biggest killer of civilians," Kubis said.