Complex sentence: The UN, with the active support of all rebel faction leaders, felt that some sort of peackeeping force would be required to uphold the ceasefire and assist the humanitarian relief effort, in conjunction with other relief agencies and NGOs.
Split into simpler sentences: The UN, with the active support of all rebel faction leaders, decided to take part in the conflict in a peacekeeping capacity. At the same time, the UN became actively involved in the humanitarian relief effort in conjunction with other relief agencies and NGOs.