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.

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), Reuters reports. "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. U.N. diplomats and officials told Reuters that one of the reasons for the slow deployment of U.N. troops to Pibor at the time was the Russian refusal to fly its helicopters there.

The head the peacekeeping mission, Hilde Johnson, told reporters after briefing the Security Council on Thursday that the mission had only "limited" helicopter capabilities in South Sudan, a country where there are few good roads.

Ban's report made clear that the violence in Jonglei state remained a serious problem and chided the government in Juba for not doing enough to protect people in the region. "The United Nations will continue to do its part, but the Jonglei crisis demonstrated that its resources are limited," the report said.

"The slow response by the government to the crisis in Jonglei state, despite the early warning provided by UNMISS, underlined ... the need for the government to assume its responsibility to forge national reconciliation as well as protect its citizens," he said. Ban called for an urgent government investigation into rights abuses in Jonglei state and for it to "bring the perpetrators to justice in order to break the cycle of reprisal attacks."

Ban warned that "hate messages" spreading throughout Jonglei threatened to spark more ethnic violence and said the government should "bring the full force of the law" to bear against those responsible for inciting violence. He called on states hosting the Sudanese diaspora to investigate those responsible for disseminating such messages.

South Sudan is also at loggerheads with Sudan over the position of their border, control of the disputed Abyei territory, and what transit fees South Sudan pays its northern neighbour to export oil from Port Sudan.

South Sudan shut down its oil fields in January to protest Khartoum's seizure of southern crude, cutting off a key source of revenue for both countries.