The number of civilians killed or injured in Afghanistan rose by nearly a quarter 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 the withdrawing American soldiers.

The Taliban continued to cause the vast majority of casualties as it stepped up its attacks this year, driving the 23 percent increase through the indiscriminate use of roadside bombs and suicide attacks in major population centers, according to the report, released on Wednesday by the United Nations Assistance Mission in Afghanistan.