Various warring groups in South Sudan are kidnapping hundreds of children and forcing them to fight, according to a new report published by an East African monitoring group. Rebel forces carried out house-to-house searches, abducting children as young as 13 in the northern villages of Kodok and Wau Shilluk in two days in early June.

The report, published by the East Africa Intergovernmental Authority on Development (IGAD), a trade bloc, outlines the abductions in detail, citing specific leaders involved in the kidnappings. The report adds to a long list of abuses that have been identified by humanitarian groups accumulated throughout the 18-month long civil war. According to U.N. estimates, there are at least 13,000 child soldiers fighting in the country.

The civil war in South Sudan began in December 2013 when President Salva Kiir accused his former deputy of planning a coup, setting off a cycle of retaliatory killings. Since then, more than two dozen armed forces have been fighting each other for control of land and resources, despite ceasefire agreements. The warring factions include government sodliers, militias backed by Ugandan soldiers and local rebel groups.

The report said Johnson Olony, a rogue former government general with a track record of forcing children to fight, joined rebel forces in May and "carried out forcible recruitment of an estimated 500-1,000 youth, many of whom were children aged between 13 and 17 years."

This is not the first time IGAD has accused Olony of recruiting children. In February, the organization published a report that his forces abducted children from Wau Shilluk. His forces were still affiliated with the government at that time, so if the allegations are proved true, he could be prosecuted for war crimes.

IGAD is just one of many international organizations to report on the widespread abuses occurring in the world's newest country. South Sudan gained independence from Sudan in July 2011 via a referendum that passed with 98.3 percent of the vote.

Last week Unicef, a United Nations agency, alleged that at least 129 children were killed during clashes between government and rebel forces in May and that an estimated 13,000 children have been recruited to fight.

“The violence against children in South Sudan has reached a new level of brutality,” Unicef Executive Director Anthony Lake said in a recent statement. “Survivors report that boys have been castrated and left to bleed to death. Girls as young as 8 have been gang raped and murdered. Children have been tied together before their attackers slit their throats. Others have been thrown into burning buildings.”

South Sudanese military spokesman Col. Philip Aguer reportedly called for a full investigation into the atrocities.

U.S. Secretary of State John Kerry said in May that the U.S. had allocated $5 million for the creation of a special court that would hold perpetrators of atrocities and other crimes in South Sudan to account, the New York Times reported.