The United Nations has urged rebels in South Sudan to release hundreds of civilians who were abducted during a surge in violence earlier this year, shortly before a peace deal was agreed with the government.

A report by the UN Human Rights Office and the UN Mission in South Sudan says 900 people were seized in the west of the country between April and August.

It documents abuses including unlawful killings, rape and sexual slavery.

A report earlier this year on fighting in the north of the country gave details of similar atrocities committed by government forces.

The two sides signed what they called a "final, final" peace deal last month to end five years of civil war.