The United States has circulated to the UN Security Council a draft resolution to impose an arms embargo on South Sudan and further targeted sanctions amid warnings by a senior UN official of possible genocide.
Political rivalry between South Sudan's President Salva Kiir, an ethnic Dinka, and his former deputy Riek Machar, a Nuer, led to civil war in 2013 that has often followed ethnic lines.
The pair signed a peace deal last year, but fighting has continued and Machar fled the country in July.
Adama Dieng, UN special adviser on the prevention of genocide, last week visited South Sudan, which gained independence from Sudan in 2011.
"I saw all the signs that ethnic hatred and targeting of civilians could evolve into genocide if something is not done now to stop it. I urge the Security Council and member states of the region to be united and to take action," Dieng told the council.
"There is a strong risk of violence escalating along ethnic lines with a potential for genocide. I do not say that lightly," he said, urging the council to impose an arms embargo.
Samantha Power, US ambassador to the UN, told the Security Council that Dieng's warning should serve as a wake-up call.
"None of us can say we did not see it coming," she said.
The Security Council has long threatened to impose an arms embargo on South Sudan, but Russia and China, which have veto powers, are sceptical whether such a move would make a difference as the country is already awash with weapons.
"We think that implementing such a recommendation would hardly be helpful in settling the conflict," Petr Iliichev, Russian deputy ambassador to the UN, said.
"Introducing targeted sanctions against South Sudanese leaders would be the height of irresponsibility now."
The Security Council set up a targeted sanctions regime for South Sudan in March 2015 and has blacklisted six generals - three from each side of the conflict - by subjecting them to an asset freeze and travel ban.
Al Jazeera's Kristen Saloomey, reporting from the UN headquarters in New York City, said violence is spreading in South Sudan, hate speech is on the rise, and the humanitarian situation is going from bad to worse.
"Against this backdrop, the Security Council is starting to discuss this latest proposal for sanctions on South Sudan," she said.
"And the people of South Sudan are looking for some signs of hope from the international community."