The United Nations says civilian casualties in Afghanistan's decade-long war have dropped by more than 20 percent in the first four months of this year as compared to the same period in 2011.

U.N. special representative for Afghanistan Jan Kubis told reporters in Kabul Wednesday that insurgents were responsible for nearly 80 percent of the 579 civilian deaths — about the same number as last year — while NATO and Afghan forces were responsible for nine percent — a drop of five percent from the last year. The cause of the remaining causalities was unattributed.

This is the first time since the United Nations starting tracking the figures five years ago that the number of civilians killed in the war has gone down. Last year, a record 3,021 civilians died as insurgents increased their use of indiscriminate weapons, including roadside bombs and suicide bombers.

Despite the drop, Kubis said Wednesday the numbers still are unacceptably high.

The U.N. report comes days after Afghan officials said a coalition airstrike in the eastern part of the country killed a family of eight, including six children. NATO is investigating the claim.

Deaths of Afghan civilians in anti-insurgency operations have been a major source of tension between President Hamid Karzai's government and the international coalition.

Also Wednesday, NATO says two of its service members were killed in southern Afghanistan, one in an insurgent attack and another in a bombing.

The independent monitoring site iCasualties.org says 175 international troops have died in Afghanistan this year.