The United Nations is pleading with members to provide military helicopters for its peacekeeping mission in South Sudan, where recent tribal violence has displaced more than 100,000 people, a U.N. report showed on Thursday.The chopper shortage arose earlier this year when Russia announced it was withdrawing all its helicopters and crew from the U.N. Mission in South Sudan (UNMISS)."I appeal to member states to make available the badly needed military utility helicopters to enhance the mission’s mobility and access to vulnerable populations,” U.N. Secretary-General Ban Ki-moon said in a report to the Security Council."In the meantime, the secretariat and UNMISS will explore other measures to enhance mobility and accessibility,” he said in the report, which the 15-nation council discussed at a closed-door meeting on Thursday.Moscow said its decision to withdraw its helicopters had nothing to do with security. But Russia’s U.N. mission has said that it was alarmed by attacks on utility helicopters operated by the Russian military for UNMISS.South Sudan declared independence in July 2011 under a 2005 peace agreement with Sudan that ended decades of civil war. But Africa’s youngest nation has been struggling to end tribal and rebel violence that has killed thousands since last year.Fighting broke out earlier this year around Pibor in Jonglei state bordering north Sudan. It remains unclear how many people were killed.