The United Nations Assistance Mission in Afghanistan (UNAMA) says more than 1,500 civilians were killed or wounded in July.
According to a report issued by UNAMA on August 3, the number of documented civilian casualties is the highest of any month since May 2017.
UNAMA said the main driver was a sharp increase in civilian casualties caused by militant attacks.
It said the use of improvised explosive devices (IEDs) accounted for more than 50 percent of the casualties.
UNAMA said it remained "gravely concerned by the harm done to civilians."
"I call on all parties not to ramp up military operations thinking that doing so will give them a stronger position in talks about peace,” UNAMA head Tadamichi Yamamoto said.
Violence in Afghanistan has spiked in recent weeks as both Afghan forces and Taliban militants attempt to increase their leverage in ongoing peace talks.
The United States and the Taliban began an eighth round of peace talks on August 3.
The UN said in a report released on July 30 that 1,366 civilians were killed between January 1 and June 30, a 21-percent decrease on the same period last year.
Air strikes and ground operations by Afghan forces killed more civilians than militant groups during that period, the report said.