(CNN) -- Afghanistan is seeing higher levels of violence this year than last year at this time, with 20% more civilians killed and the number of "security incidents" up by 66%, the United Nations says in a new report.

The number of civilians killed by the United States and its allies was lower, but insurgent attacks are significantly higher, meaning the overall number of civilian deaths is up.

More than 2,400 civilians were killed, and more than 3,800 injured in the first 10 months of this year, the report says.

More than three out of four of the casualties were caused by "anti-government elements," it says.

That's a 25 percent increase on last year, it said. Deaths caused by U.S. troops and their allies were down 18 percent.

"Assassinations" of civilians and police reached "unprecedented numbers" in August, and there were an average of three suicide attacks per week, the U.N. said in its quarterly report.

The United Nations report, which was finalized December 10, also tracked widespread kidnapping of aid workers.

A total of 134 were abducted between June and the end of October, the U.N. said. Most were freed quickly, but one Afghan was killed by his captors, the world body said.

A British aid worker was also killed this year by U.S. special forces trying to rescue her from kidnappers. Linda Norgrove, 36, was killed by an American grenade thrown by a rescuer who did not realize she was there.

U.S. Gen. Stanley McChrystal, who was the top international military commander in Afghanistan until he was fired in June, urged his troops to avoid civilian casualties in order to avoid making enemies of the population.

"It is better to miss a target than to cause civilian casualties," McChrystal said in December 2009. "We can always target enemy leaders later. We can't make up for the fact that we killed civilians."

But there continued to be high-profile incidents, including one where 23 civilians were killed in a NATO airstrike in February.

McChrystal personally apologized to Afghanistan's President Hamid Karzai for those killings and ordered an investigation. It concluded that troops needed more training.

McChrystal's dismissal was not related to the incident. He was fired over disrespectful comments his inner circle made about top White House officials in a profile of the general in Rolling Stone magazine.

The United Nations report was prepared for Secretary-General Ban Ki-moon.