Last year was the deadliest for civilians in the decade-long U.S.-led war in Afghanistan, according to a U.N. report released Saturday.

The report said 3,021 civilians were killed in 2011, an 8 percent increase from 2010. It was the fifth consecutive year that the number of deaths has increased.

Insurgents were responsible for the vast majority of the casualties, at least 2,332, according to the report. Most of the victims were killed by makeshift explosives or suicide bombers. The use of both tactics increased sharply over the past 12 months.

The report describes a deteriorating security situation as NATO’s war effort begins to ebb, and as the United States pursues negotiations with an insurgency that shows no sign of relenting. Meanwhile, Western officials have described the Taliban’s power as waning, pointing to a 20 percent decrease in the number of coalition troops killed last year.

But the U.N. report suggests that the number of foreign troops killed is a poor indicator of the Taliban’s ability to foment unrest.

Even if the insurgents’ ability to fight conventional forces has diminished, they remain a major source of instability in the country’s southern and eastern provinces, where deadly attacks often target civilians. Suicide bombings killed 410 civilians last year, 80 percent more than in 2010. U.N. officials called such shifts “changes in the tactics of the parties to the conflict.”

The report attributed about 400 of the deaths to NATO and Afghan forces, a small decrease from 2010, with aerial attacks responsible for about half of those casualties. NATO-led night raids, a long-standing source of tension between the United States and President Hamid Karzai, accounted for 63 deaths, down 22 percent from the previous year.

“Afghan children, women and men continue to be killed in this war in ever-increasing numbers,” said Jan Kubis, U.N. secretary-general’s special representative for Afghanistan. “For much too long, Afghan civilians have paid the highest price of war.”

Gen. John R. Allen, the top commander in Afghanistan, said the decline in NATO-related deaths was a validation of the coalition’s efforts to reduce civilian casualties. “Every citizen of Afghanistan must know [the International Security Assistance Force] will continue to do all we can to reduce casualties that affect the Afghan civilian population,’’ he said in a statement. “This data is promising but there is more work to be done.”

The Taliban issued several public statements in 2011 related to its concern for civilian casualties, claiming at one point that “strict attention must be paid to the protection and safety of civilians during the spring operations.”

As Afghan forces prepare to inherit the conflict, questions remain about their ability to minimize civilian casualties. In the last six months of 2011, the United Nations documented 41 civilians killed and 133 injured by Afghan soldiers and police — a 192 percent increase in civilian deaths and a 55 percent increase in civilian injuries compared with the same period in 2010.

The conflict last year also had an unprecedented impact on refugees, forcing 185,632 Afghans from their homes in 2011, an increase of 45 percent from the previous year.