The organisation says 3,545 civilians were killed and 7,457 injured last year
The United Nations says there were over 11,000 casualties in Afghanistan last year - including more than 3,000 civilian deaths.
The figures are the highest recorded in the UN's annual report on civilians in conflict since the organisation began systematic documentation of casualties in 2009.
3,545 civilians were killed and 7,457 injured, including an "unprecedented number of children", according to the United Nations Assistance Mission in Afghanistan (UNAMA).
The international human rights organisation says the damage being inflicted on people in the turbulent country is unacceptable.
Including Taliban-claimed attacks, UNAMA assigned responsibility for 62% of total civilian casualties in 2015 to anti-government elements.
17% of the civilians deaths and injuries were caused by pro-government forces.
Nicholas Haysom, head of UNAMA, said, "the real cost we are talking about in these figures is measured in the maimed bodies of children, the communities who have to live with loss, the grief of colleagues and relatives, the families who have to make do without a breadwinner, the parents who grieve for lost children, the children who grieve for lost parents".
UN High Commissioner for Human Rights, Zeid Ra'ad Al Hussein "the people of Afghanistan continue to suffer brutal and unprincipled attacks that are forbidden under international law. The perpetrators of the violations, documented by UNAMA and my staff, must be held to account".