KABUL, Afghanistan — The number of civilians killed or wounded in Afghanistan rose by 23 percent in the first six months of 2013, according to a United Nations report on civilian casualties, reversing a decline last year and signaling the challenge Afghan forces face as they take over all combat duties from American soldiers.

The Taliban continued to cause the majority of casualties, stepping up violence through the indiscriminate use of roadside bombs and suicide attacks in major population centers, according to the report, released Wednesday by the United Nations Assistance Mission in Afghanistan.

With the American pullout planned for the end of 2014, responsibility for the fighting is increasingly falling to the Afghans. Aware of the security transition in place, the insurgency has stepped up its efforts to strike the Afghan forces, hoping to undermine confidence in government institutions. That effort has included a focus on complex and daring attacks in major cities like the capital, Kabul, where Afghans often find themselves in harm’s way.

In many areas where the international forces have shut down bases or otherwise reduced their footprint, the fight for control between the Afghan forces and the insurgency has intensified — with women and children often bearing the brunt of the violence. Fighting between the two groups over such areas of the country was the second biggest cause of civilian deaths and injuries.