The report put a spotlight on “complex attacks,” a type of suicide assault that is becoming more deadly, United Nations officials say. These involve two or more commandos with suicide vests seizing a building or taking hostages, fighting for hours and detonating their explosives only when security forces close in.

Such attacks have recently become a hallmark of the war. The ensuing mayhem sends a clear message: that the Taliban can strike at will, even in ostensibly safe cities like Kabul, where the “ring of steel” security cordon around the capital has been looking more like a colander.

Deaths from complex attacks and other suicide bombings rose to 605 last year from 398 in 2016, according to the report, with the highest number of civilian casualties in Kabul.

One of the deadliest single episodes of the war for civilians took place last May, when a gigantic truck bomb estimated to contain two metric tons of high explosives went off on a Kabul street during rush hour, killing 92 and wounding 491 people.

The Taliban have also stepped up assassinations in the countryside, targeting doctors delivering polio vaccinations, workers on demining teams and religious leaders who preach against the group, among others, the United Nations said.

The report blamed the Islamic State’s Afghan branch for 1,000 civilian casualties — 399 killed and 601 wounded.

In a statement that the United Nations included with the report, the Taliban denied targeting civilians and blamed the United States and its allies for waging war in Afghanistan in the first place. “For the past 17 years, hundreds of thousands of innocent Afghans were killed after the influx of foreigners, and they are still being killed,” it said.