The U.N. chief in South Sudan Friday called for an immediate release of 12 colleagues seized by rebel forces, warning that their capture was a possible war crime. Roughly 100 rebel fighters, who have been battling the government for nearly two years, captured 30 members of the U.N. peacekeeping mission in South Sudan (UNMISS) Monday. While 18 Bangladeshi peacekeepers were later freed, 12 South Sudanese working for UNMISS are still being held. "I am adamant that the taking of UNMISS personnel as hostages is equivalent to an attack on the U.N.," Ellen Margrethe Loj said. "Such attacks against U.N. peacekeepers and other personnel may constitute a war crime." About 12,500 peacekeepers are deployed in South Sudan, which has been experiencing conflict since December 2013.