KABUL - The accidental killing of a cousin of President Hamid Karzai by a U.S. Special Forces team became on Thursday the latest in a string of high-profile civilian casualties that have further soured the relationship between the president and his American benefactors.

The fatal shooting late Wednesday of Yaar Mohammad Khan, 65, a second cousin of the president, in a night raid in southern Afghanistan, stoked fresh anger from Karzai, who has become increasingly critical of the way NATO is prosecuting the war.

"The president sees this as another lamentable case of loss of life," said presidential spokesman Waheed Omer. "This was the result of an irresponsible and unnecessary night raid. His message is that they have to stop these irresponsible night raids that kill innocent Afghans."

Civilian deaths caused by NATO have declined over the past year, even as the tempo of the war has dramatically increased and thousands of new U.S. troops have poured into Afghanistan, according to military and U.N. statistics.

Still, U.S. military officials say the prominent nature of several recent incidents could severely undermine the counterinsurgency effort here as the war continues in its 10th year.

The incident comes a little more than a week after NATO pilots in eastern Afghanistan mistakenly killed nine boys collecting firewood. The airstrike prompted an unusually irate statement from Karzai and a rare personal apology from Gen. David H. Petraeus, the commander of U.S. and NATO troops here. It also provoked demonstrations by Afghans angry over what is perceived as disregard among foreigners for Afghan lives.

"Although perception is always hard to overcome, here it is a different challenge because of illiteracy and lack of trust of foreigners," a U.S. military official said Thursday night, speaking on condition of anonymity to discuss a politically sensitive issue.

Since late 2009, NATO has taken several steps to reduce civilian casualties, including placing more restrictions on when troops can fire their weapons. The effort led to a 26 percent drop last year in the number of civilians killed by NATO or Afghan government forces, according to a U.N. report issued Wednesday.

By contrast, the number of civilians killed by the Taliban and its allies jumped 28 percent. The U.N. estimates that insurgents were responsible for three-quarters of the 2,777 civilians killed during fighting in Afghanistan last year. Overall, civilian deaths were up 15 percent in 2010, largely because of Taliban suicide bombings, roadside bombings and assassinations.

Khan's death late Wednesday garnered immediate attention here, with the Taliban issuing a statement noting that not even Karzai's family is spared the "evil" of NATO forces.

Relatives said U.S. Special Operations troops were apparently attempting to take Khan's son into custody in the family's home in Dand district of Kandahar province, suspecting he was a Taliban fighter. For reasons that have not been fully explained, the 65-year-old was fatally shot.

"The target was someone else, but unfortunately my poor cousin became the victim," said Ahmed Wali Karzai, the president's half-brother.