The UN Security Council has authorised a 4,000-strong regional protection force for South Sudan's capital, Juba.
These African troops will have a more robust mandate than the 12,000 UN soldiers already in the country.
South Sudan's government has said it opposes the move, despite the UN resolution threatening an arms embargo if it blocks the new deployment.
Fighting between rival forces in July left hundreds of people dead less than year after a peace deal was signed.
The UN force struggled to keep civilians safe as more than 35,000 people rushed to its base in Juba.
More than 100,000 South Sudanese have now fled to neighbouring countries in the wake of the latest unrest,
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Unlike the peacekeeping force in place, the African force, which will be under the auspices of the UN mission, will be able pro-actively to engage those threatening civilians.
But it is not clear how the mission will be able to go ahead without South Sudan's co-operation.
The US-drafted resolution says it will "use all necessary means, including undertaking robust action where necessary, and actively" patrol to enforce peace.
It will also ensure the protection of Juba and the airport and "promptly and effectively engage any actor that is credibly found to be preparing attacks or engages in attacks".
When African leaders
, they said the soldiers were likely to come from Ethiopia, Kenya, Rwanda, Sudan and Uganda.
The African Union explained the mission would be similar to the deployment of a 3,000-strong special force that took on and swiftly defeated the M23 rebels in the Democratic Republic of Congo in 2013.
In this case the rival forces are not rebel groups - but rival sides in a unity government recently formed after a two-year civil war.
In July it was bodyguards for then Vice-President Riek Machar and President Salva Kiir's presidential guards who fought each other, sparking days of violence.
Political differences between the two men ignited civil war in December 2013 - and they only agreed to settle their differences under intense international pressure, signing a peace deal last August.
Mr Machar, who only returned to Juba in April, fled the city during the latest fighting, demanding the deployment of a neutral force to keep the peace and guarantee his safety.
Correspondents say this new African force appears to meet these demands but Mr Kiir removed Mr Machar as vice-president a few weeks ago and his government opposes the deployment of the force.
The success or failure of the protection force is contingent on the ability of the international community to compel South Sudan to comply, argues UN analyst Mark Leon Goldberg.
"This new resolution will be only a fig leaf of a solution unless diplomatic pressure is brought to bear on the government of South Sudan sufficient to convince it to co-operate with this new force,"
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South Sudan became independent from Sudan in 2011 but its short history has been marred by civil war.