The council narrowly adopted a US-drafted resolution with the backing of nine countries. Russia, China and four other countries abstained.

The United States had lobbied hard to win votes for the arms embargo after it failed in 2016 to persuade the council to back the measure and cut off the arms flow to South Sudan.

US Ambassador Nikki Haley told the council that support for the arms embargo will send a message to South Sudan's leaders that "we are fed up with delays and stalling."

"These are the weapons that armed groups used to shoot fathers in front of their wives and children, to hold up convoys of food aid, or to assault women and girls," Haley told the council ahead of the vote.

A draft resolution requires nine votes and no veto to be adopted in the 15-member council.

South Sudan won independence from Sudan in 2011, with critical backing from the United States, which remains Juba's biggest aid donor.

Countless efforts have failed to bring peace to the country now in its fifth year of a war that has seen ethnic killings, gang rapes and other atrocities documented by UN rights officials.

"Armed groups in South Sudan are literally burning people alive and hanging them from trees. This is barbaric," Haley said.

- Six countries abstain -

Ethiopian Ambassador Tekeda Alemu, whose country is leading a regional peace effort, opposed the arms embargo, arguing that it could undermine the fragile diplomacy at work.

"Without prudence and patience whatever human rights violations there are today, they could be even worse. Averting the worse is our objective," Alemu told the council.

Aside from Russia and China, Ethiopia abstained along with Equatorial Guinea, Kazakhstan, Bolivia.

Many of the countries that abstained argued that the African Union and the regional IGAD grouping did not support the punitive measures and had appealed for more time to allow peace efforts to yield results.

The resolution expressed "deep concern at the failures of South Sudan's leaders to bring an end to the hostilities" and imposes the arms embargo until May 2019.

Former military chief Paul Malong and Malek Ruben Riak, a former deputy chief of general staff for logistics, were added to the UN sanctions blacklist, hit by a global visa ban and assets freeze.

South Sudan descended into civil war in late 2013 when President Salva Kiir accused former vice president Riek Machar of plotting a coup.

In the years since, tens of thousands have been killed and millions have been uprooted. Seven million South Sudanese, more than half of the population, are in need of food aid, according to the UN.

Led by Ethiopia, efforts have intensified over recent weeks for talks between Kiir and Machar but the latest ceasefire agreed by the sides failed to take hold.

South Sudan's parliament voted this week to allow Kiir to remain in power until 2021, a move that will complicate negotiations with Machar on a power-sharing deal.

Diplomats said Ivory Coast and Kuwait, two non-permanent members, came under strong US pressure to back the arms embargo, with Washington appealing directly to the governments in their capitals.

"The US has tried to achieve this arms embargo for several years, when the last administration brought forward a resolution that did not pass," said a Security Council diplomat.

"I think Ambassador Haley's new approach definitely helped to amass support for the embargo this time. It's quite an important step forward for the Council and the South Sudanese people," he added.