(Reuters) - Government and rebel forces in South Sudan abducted hundreds of women and girls this year and many have been raped and forced into sexual slavery, the U.N. mission to the country said on Tuesday.

Other young people were forced to become child soldiers, according to a report by the U.N. Mission in South Sudan, which said that many of those abducted remain in the hands of their captors.

The abuses were committed during a civil war by forces loyal to the government of President Salva Kiir and rebels fighting for Riek Machar. The abductions violate international law and may amount to war crimes, the report said.

“The girls are sometimes only 12 years old and were chosen as wives for the military. They had to parade in front of them and they (soldiers) could choose whomever they wanted. They used them and of course they were raped and (subjected to) sexual slavery,” the report said.

Army spokesman Lul Ruai Koang denied the allegations.

“We have no evidence showing that we have committed all the allegations levelled against us. What we have to tell them: they should bring the evidence, not only the report from Geneva and New York,” he said.

Rebel spokesman Lam Paul Gabriel said: “The leadership of the SPLA (Sudan People’s Liberation Movement/Army) will investigate these accusations through a right channel and bring anyone found guilty to book so that justice is served”.

The report’s publication followed an investigation between April and August in the southwest of the country.

South Sudan’s civil war began in 2013. The two sides signed a peace treaty last month. Around 4 million people have been displaced from their homes during the conflict.