Overall 581 civilians were killed and 1,192 injured in Afghanistan between January and March, down nearly a quarter from a year earlier, and at the lowest level since 2013.
That fall was largely driven by a reduction in suicide attacks, the UN said.
Pro-government forces caused 305 civilian deaths, and opposition fighters 227 deaths. Another 49 deaths were caused by crossfire or could not be attributed.
The US' Afghanistan peace envoy, Zalmay Khalilzad, tweeted that he was "distressed by reports of civilian casualties" but did not directly address the UN figures.
Khalilzad, who has been tasked with brokering a deal with the Taliban, added that the only way to stop the blood flow was a ceasefire.
Afghan President Ashraf Ghani's government blamed insurgents for causing the deaths by using human shields.
"The real reason behind the number of civilian casualties is armed militants who use civilians as their shield and use their homes as their battleground," the presidential information coordination centre said on Twitter.
The report also detailed attacks on education and healthcare sites.
The UN recorded 18 instances in which schools were targeted, the majority by Taliban or other insurgents. Among the most serious were attacks on girls' schools in Farah province, setting buildings and equipment on fire.
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