The United Nations' top human rights official in South Sudan says no one has been detained in connection with recent killings of civilians in the western town of Wau.

Eugene Nindorera in a statement Sunday says he was "shaken" to hear testimonies of victims of the government offensive that killed at least 16 people and displaced more than 23,000. Residents said soldiers singled out civilians of the Fertit and Luo ethnic groups in retaliation for a rebel attack on government forces.

Nindorera says the continued impunity is one of the biggest challenges to stopping violence in South Sudan's civil war, which has entered its fourth year.

The African Union and South Sudan's government have long promised to create a hybrid court to try leaders for war crimes.