KABUL: In a report, the international body said pro-government forces were responsible for 54% of civilian deaths during the first three months of 2019.
Tadamichi Yamamoto, the UN's special representative for Afghanistan, called the latest figures "shocking".
The findings come as civilian casualties fell to their lowest level for the period since 2013.
Both the US and the Taliban have held peace talks since October last year, but fighting has continued as both sides look for leverage in negotiations.
The UN, which has been monitoring the conflict since 2009, says between January and March insurgent groups killed 227 civilians and injured 736 others.
In contrast, Afghan and international forces caused 305 deaths and 303 injuries - a 39% jump from the same period last year.
The leading causes of casualties, according to the UN, were ground fighting and improvised explosive devices (IEDs).
Air strikes also killed 145 civilians during the period, half of them women and children. The UN attributed almost all of these to American forces.
Mr Yamamoto said "all parties must do more to safeguard civilians".
Overall, however, the UN reported a 23% drop in casualties compared with the same period in 2018. The agency said it was "unclear" whether this was influenced by the peace talks or "any measures taken by parties to the conflict to better protect civilians".
US forces have been stationed in the country since it led a military coalition to overthrow the Taliban in 2001.
Around 14,000 American troops are helping to train Afghan security forces.
The Taliban, however, currently control and influence more territory than they have at any point in the past 18 years.