The United Nations secretary-general, Ban Ki-moon, has said he fears genocide could start in South Sudan unless immediate action is taken, renewing his plea for the security council to impose an arms embargo on the country.

“If we fail to act, South Sudan will be on a trajectory towards mass atrocities,” Ban told the security council.

Noting that the UN’s special adviser on the prevention of genocide, Adama Dieng, has described genocide as a process, Ban said: “I am afraid that process is about to begin unless immediate action is taken.

“The security council must take steps to stem the flow of arms to South Sudan,” he added.

Political rivalry between Salva Kiir, South Sudan’s president, 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 shaky peace deal last year but fighting has continued. Machar fled the country in July.

“Reports suggest that President Salva Kiir and his loyalists are contemplating a new military offensive in the coming days against the [Machar-allied opposition troops],” Ban said. “Moreover, there are clear indications that Riek Machar and other opposition groups are pursuing a military escalation.”

The US has been struggling to secure the minimum number of votes needed for the security council to impose an arms embargo on South Sudan. In order to be adopted, a resolution needs nine votes in favour and no vetoes.

Diplomats have said that so far only seven members were in favour, with the remaining eight planning to abstain or vote no. While Russia and China, two of the five security council members with veto power, are skeptical of whether an arms embargo would achieve much in a country awash with weapons, diplomats did not expect them to block the measure.

“We think it’s extremely important to vote on this by the end of the year,” said the US ambassador to the United Nations, Samantha Power.

South Sudan, a small oil producer that remains one of the world’s poorest nations, gained independence from Sudan in 2011.

The slide into conflict has left many of the nation’s 11 million people struggling to obtain enough food.

UN peacekeepers have been deployed in South Sudan since 2011. There are some 13,000 UN soldiers and police in the country.

“South Sudan is on the brink,” UN aid chief Stephen O’Brien warned the council. “How many more clues do you, do we all, need to move from our anxious words to real preventative action?“