By Michelle Nichols

UNITED NATIONS, March 19 (Reuters) - A South Sudanese warlord who abducted about 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, United Nations global education envoy Gordon Brown said on Wednesday.

"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," Brown told reporters at the United Nations.

Brown was referring to the kidnap of more than 200 girls by Boko Haram militants from Chibok, Nigeria. Boko Haram, which is fighting to carve an Islamic state in northeast Nigeria, attacked the girls' secondary school almost a year ago at exam time. The abduction led to a global campaign to find them.

In South Sudan, the U.N. Children's Fund UNICEF said in February that gunmen kidnapped the boys in oil-rich Upper Nile State while they were sitting exams.

Brown, a former British prime minister, said they were aged between 12 and 15. Brown was starting a campaign for the world to guard schools from military use and attacks and give them the same protections as Red Cross hospitals.

"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," Brown said. He did not give details on who had taken the boys.

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

South Sudan declared independence from Sudan in 2011. In January, the United Nations 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," Brown said.

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 has reopened ethnic fault lines that pit Kiir's Dinka people against Machar's ethnic Nuer forces.

At least 10,000 people have been killed and 1.5 million civilians displaced. On Tuesday, U.S. Ambassador to the U.N., Samantha Power, said a lack of accountability for atrocities hinders a bid for peace. (Reporting by Michelle Nichols; Editing by Grant McCool)