US Ambassador Samantha Power made the case for cutting off the weapons flow to the war-torn country during a council meeting, arguing that all sides were mobilizing for more fighting and that action was needed to prevent mass atrocities.
"The situation is not getting better, but worse, and we are sitting on our hands," Power told the council yesterday. "Large-scale attacks could start at any moment."
The United States last month presented a draft resolution on imposing an arms embargo following UN warnings that the war-torn country could descend into genocide.
The United States will request a vote on the arms embargo before the end of the year, Power said.
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UN Secretary General Ban Ki-moon backed the US call to ban arms deals with South Sudan, saying an embargo would reduce the capacity of all sides to wage war.
"If we fail to act, South Sudan will be on a trajectory towards mass atrocities," said Ban.
France and Britain support an arms embargo, which could come up for a vote as early as Thursday, according to diplomats.
Japan has argued that imposing an arms embargo would antagonize President Salva Kiir's government and put peacekeepers' lives at risk in a conflict already marked by horrific levels of brutality.
Russian Deputy Ambassador Petr Iliichev cast doubt over UN warnings of a risk of genocide, arguing that criminal groups and "undisciplined" troops were responsible for mass violence, and not the government's policy.
"We would refrain from concluding that there is any form of targeted systemic policy," he told the council.
More than 6 million people -- half of South Sudan's population -- are in need of urgent aid and humanitarian organisations expect this number to rise by 20 to 30 per cent next year, O'Brien told the council.
Some 1.3 million South Sudanese have fled across borders as refugees, including 383,000 who have fled to Uganda since July, according to UN figures.
The world's youngest nation, South Sudan descended into war in December 2013, leaving tens of thousands dead and more than 3.1 million people displaced.