There were 11,002 civilian casualties in 2015 including 3,545 deaths, the UN said in its annual report on Afghan civilians in armed conflict, a four percent rise over the previous high in 2014.
"The harm done to civilians is totally unacceptable," said Nicholas Haysom, the UN's special representative for Afghanistan.
"We call on those inflicting this pain on the people of Afghanistan to take concrete action to protect civilians and put a stop to the killing and maiming."
The UN began compiling the annual report in 2009.
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Including Taliban-claimed attacks, the United Nations Assistance Mission in Afghanistan assigned responsibility for 62 percent of total civilian casualties in 2015 to anti-government elements.
The Taliban rejected the report's findings in a statement today.
But the report also noted a 28 percent year-on-year surge in the number of casualties caused by pro-government forces, including the Afghan army and international troops.
The report criticised Afghan forces in particular for their reliance on explosives in populated areas.
US and other international troops moved from a combat role to a training, advisory and assistance role in Afghanistan on January 1, 2015, leaving Afghan forces to take the lead in fighting the resurgent militants as they targeted towns and cities.
"Why did they fire this rocket? Why was it necessary?" the father of a man killed in shelling by the Afghan army in a village in Wardak province in December was quoted as saying in the report.
"Can you imagine how difficult it is when your son is lying in his own blood and you are crying for him?" the father is quoted as saying.
The Afghan government thanked UNAMA for the report in a statement released today, though it disputed the decision to attribute such a large number of civilian deaths to unknown factors rather than the Taliban.