The UN Security Council rejected a US-drafted resolution on imposing an arms embargo and more sanctions on South Sudan in a setback to proponents who said the measure could help mitigate a four-year-long conflict that some UN officials warned could escalate into a genocide.

The measure garnered only seven votes in favour in the 15-member council on Friday, while eight countries including Russia, China and Japan abstained. Nine votes and no veto are required for resolutions to be adopted by the council.

The United States, backed by Britain and France, had argued that cutting off the arms flow was urgently needed following UN warnings of a risk of mass atrocities.

But Russia, China, Japan, Malaysia, Venezuela and, more importantly, the three African council members - Angola, Egypt and Senegal - were not swayed and they all abstained.

"The council members who didn't support this resolution are taking a big gamble that South Sudan's leaders will not instigate a catastrophe," US ambassador Samantha Power told the council after the vote. 

"It is the people of South Sudan who will pay an unbearable price."

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.

 

On Monday, UN Secretary-General Ban Ki-moon told the Security Council that he feared genocide was about to start in South Sudan unless immediate action is taken, renewing his months-long plea for an arms embargo.

South Sudan's UN ambassador, Akuei Bona Malwal, said such descriptions were exaggerated and did not "reflect the reality on the ground". Earlier this month South Sudan's President Salva Kiir denied reports that ethnic conflict was so severe in the country that it risked escalating to genocide.

The outcome of the vote was a setback for the US, which helped South Sudan gain independence in 2011, but has been unable to steer the country away from a war that erupted two years later.

Power also said the US was prepared to push for another vote on an arms embargo in the future.

Aside from the arms embargo, the US had sought to put rebel leader Reik Machar, army chief Paul Malong and information minister Michael Makuei on a sanctions blacklist, subjecting them to an assets freeze and a global travel ban.

A joint statement issued by seven non-government organisations, including Human Rights Watch, Amnesty International, and The Enough Project, has condemned the UN's failure to adopt the resolution.

"South Sudanese civilians had a reasonable expectation that the Security Council would make good on its long-standing threat to impose an arms embargo and extend sanctions to some of the senior leaders who have been responsible for grave human rights abuses," said John Prendergast, founding director of the Enough Project. "I can only imagine their frustration with today's vote."

The statement cited how the African Union and UN investigators have documented war crimes, including killings and rape of civilians, and forced recruitments of child soldiers by opposing groups.

"In the last few months, there has been an increase in incitement to violence, hate speech by senior leaders, and targeting of civilians, sometimes based on ethnicity, in parts of the country that were previously untouched by the civil war," the statement said.

"The Security Council had an opportunity to show that it stands with the civilian victims of this conflict," said Akshaya Kumar, deputy United Nations director at Human Rights Watch.

"Instead, this failure gives the warring parties in South Sudan a green light to buy more weapons and materiel that will end up being used against civilians."

On the other side, Japan, which has some 350 troops serving in the UN mission in South Sudan, had warned the resolution would have antagonised Kiir's government and put peacekeepers' lives at risk.

Opponents of the sanctions pointed to Kiir's call earlier this month for a national dialogue to restore peace, saying that initiative must be given a chance.