A South Sudanese warlord who abducted 89 boys from their school has offered to let them return to finish exams as long as they are then given back to him to fight.

The UN children's fund UNICEF said gunmen kidnapped the boys in the oil-rich upper Nile state while they were sitting exams in February.

UN envoy Gordon Brown, the former UK prime minister, said the boys were aged between 12 and 15.

"The latest information is that the terrorist group has offered to allow them to sit their exams as long as they can then take them back as child soldiers," Mr Brown said.

He did not give details on who had taken them.

South Sudan declared independence from Sudan in 2011. In January, the UN secured the release of about 3,000 child soldiers.

"About 12,000 children in South Sudan have been abducted in recent times by different factions ... who are training these children who have been abducted as soldiers for the future," Mr Brown said.

"The whole world should be protesting as we did over Chibok about any child that is abducted from their school and any child that has been kidnapped ... as is happening to so many children in South Sudan," he said in reference to the kidnap of more than 200 girls by Boko Haram militants in Nigeria.

Mr Brown was helping start a campaign for the world to guard schools from military use and attacks and give them the same protections as Red Cross hospitals.

He said there had been more than 10,000 attacks on schools worldwide in the past five years and that 28 million children in areas of conflict or emergency were unable to attend school.

South Sudan plunged into civil war in December 2013 when a political crisis sparked fighting between forces loyal to president Salva Kiir and rebels allied with his former deputy Riek Machar.

The conflict reopened ethnic fault lines that pitted Kiir's Dinka people against Machar's ethnic Nuer forces.

At least 10,000 people have been killed and 1.5 million civilians displaced.

This week the US ambassador to the UN, Samantha Power, said a lack of accountability for atrocities hindered a bid for peace.

Reuters