South Sudan’s government said Friday it was pulling out of peace talks to end a 20-month long civil war after rebel forces split despite international threats of sanctions.

Tens of thousands of people have been killed in a war marked by widespread atrocities on both sides, and diplomats warned the collapse of the latest peace efforts could trigger "serious consequences” for the rival leaders.

"We suspend the peace talks until the two rebel factions sort out their differences,” top government official Louis Lobong said, after meetings with President Salva Kiir.

South Sudan’s civil war began in December 2013 when Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that has split the landlocked country along ethnic lines.

Regional mediators, backed by US President Barack Obama during his recent visit to Ethiopia, gave Kiir and Machar until August 17 to halt the civil war.

On Tuesday, however, top rebel generals said they had split from Machar, accusing him of seeking power for himself, and adding they would not recognise any deal agreed.

Obama has warned Kiir and Machar that if they failed to strike a deal the US will "move forward with a different plan, and recognise that those leaders are incapable of creating the peace that is required.”

Massacres and rape

The war has been characterised by ethnic massacres and rape. Recent attacks have included castration, burning people alive and tying children together before slitting their throats.

Over 70 per cent of the country’s 12 million people need assistance, while 2.2 million people have fled their homes the UN says, with areas on the brink of famine.

Possible punitive measures could include an arms embargo and targeted sanctions including travel bans and asset freezes.

The latest round of talks opened on August 6, mediated by the regional eight-nation bloc IGAD, the Intergovernmental Authority on Development, as well as the United Nations, African Union, China and the "troika” of Britain, Norway and the United States.

Envoys have said international patience has run out.