KABUL—Civilian casualties in Afghanistan are at a high, a United Nations report says, in a grim reminder that fighting in the country is as intense as it has ever been since the U.S.-led invasion of 2001.

The U.N. said 1,592 civilians were killed and 3,329 injured in the first half of 2015. The figures are slightly higher than for the period in 2014, which was the most violent year since 2001. The findings of the U.N. report, issued Wednesday, are an early indication that 2015 could be even bloodier.

The data coincide with a change in the dynamics of the war in Afghanistan. The U.S. and its coalition partners ended their combat mission at the end of last year—a step that U.S. President Barack Obama described at the time as a responsible end to America’s longest war. Since January Afghan forces have been fighting almost on their own.

The war is far from over for Afghans. In recent months, fighting between the central government’s forces and the Taliban has moved closer to populated areas, as insurgent fighters tried—mostly unsuccessfully—to capture district centers and other territory. There has been heavy fighting in provinces ranging from Kunduz in the north, Helmand in the south and Nangarhar in the east.

The U.N. report says the leading cause of civilian deaths and injuries is ground fighting between Afghan troops and antigovernment forces. Other causes of civilian casualties are the Taliban’s continued use of roadside bombs and suicide attacks, a tactic that has become more common this year.