The statement by Susana Malcorra, undersecretary-general of the UN Department of Field Support, came during a recent increase in tribal violence in a remote area of South Sudan that caused some 60 000 people to flee.
"My sense is that at this point in time, Russia is seriously considering whether to stay or to leave South Sudan," Malcorra told Reuters in an interview.
The departure of Russia, which provides key services for the UN Mission in South Sudan, or UNMISS, could complicate matters in Africa's newest country, an impoverished oil-producing state struggling to establish a functioning security sector that is under the control of the Juba government.
Until recently, Russia had eight helicopters that were being used by UNMISS, Malcorra said. After its utility helicopters were attacked by South Sudanese security forces last autumn, Russia decided in December to withdraw four of them and now appears to be contemplating the withdrawal of the other four, she said.
Russia's UN mission acknowledged that Moscow was concerned about the poor security in South Sudan but said there was no point in speculating about its future plans with UNMISS, which has a mandated full strength of 7 000 military personnel, while negotiations with the United Nations were under way.
In a statement to Reuters, the mission said Russia was "alarmed" by attacks on utility helicopters operated by the Russian military for UNMISS.
"Recently the situation in providing security to the Russian helicopter crews has been deteriorating," the mission said.
"Administrative matters pertaining to a new letter of assist [contract with the UN] are being discussed by the parties and any speculation about Russia's participation in the UN mission for the future is inappropriate," the spokesperson said.
South Sudan declared independence in July under a 2005 peace agreement with Khartoum that ended decades of civil war. But the nation has been struggling to end tribal and rebel violence that killed thousands last year.
Fighting broke out recently between members of the Lou Nuer tribe and the rival Murle tribe. Some 6 000 armed Lou Nuer members attacked the town of Pibor in Jonglei state bordering north Sudan. It remains unclear how many people were killed.
After the Lou Nuer campaign lasting several days, Murle men attacked two villages in Akobo county in northern Jonglei, killing at least 24, according to the government.
The UN diplomats and officials told Reuters that one of the reasons for the slow deployment of UNMISS troops to Pibor at the time of the clashes was the Russian refusal to fly its helicopters there.
Russia, the UN officials say, has been refusing to fly its helicopters without a new "letter of assist" to replace the previous one, which expired at the end of November. But Malcorra said the real reason Russia had grounded its choppers appeared to be security, not administrative bottlenecks.
If a country's contract for providing military assets with a UN mission expires, the expired contract traditionally remains in force until a new one is signed, Malcorra said.
Military air services
"Even though there are administrative hurdles, I do not believe the administrative hurdles are the cause of this problem," she said. "I believe it was a safety and security ... question."
One senior UN official, who spoke to Reuters on condition of anonymity, was highly critical of Russia, saying the grounding of its helicopters was "outrageous" and that the UN peacekeepers needed to be prepared to put up with a certain amount of risk in the interest of protecting civilians.
Malcorra was careful not to criticise Moscow, which provides civilian and military air services for many UN missions worldwide.
"It is clear that the reason why Russia has [grounded] the helicopters is based on the threat and the risk the troops have faced," she said. "And I can fully understand that."
South Sudan's government has apologised to Russia for the attacks on its helicopters last year and vowed to take steps to ensure there were no new attacks on Russian aircraft by South Sudanese. The UN Secretary-General Ban Ki-moon has also spoken with senior Russian officials, as has Malcorra.
To cover for the shortage of helicopters in South Sudan, Malcorra said UNMISS would be temporarily using helicopters from the UN mission in Democratic Republic of the Congo and a separate Ethiopian stabilisation force, called UNISFA, currently in the disputed Abyei region bordering north and South Sudan.