US Secretary of State John Kerry said Friday he had secured promises from South Sudan's president and rebel leader to hold direct talks on ending the country's brutal four-month-old civil war.

After talks in Juba with President Salva Kiir, Kerry said the conflict -- which has been marked by widespread atrocities and war crimes by both sides -- could not be allowed to rage on amid warnings of a slide towards genocide and famine.

"I told President Kiir that the choices that he and the opposition face are stark and clear,” Kerry said.

"The unspeakable human costs that we have seen over the course of the last months are unacceptable to the global community,” he added. "There could be major famine in the course of months ahead.”

Officials said Kerry has brandished the threat of targeted sanctions against Kiir and rebel leader Riek Machar, and the top US diplomat said the two sides had for the first time agreed to sit down for face-to-face talks in neighbouring Ethiopia.

Kerry said President Kiir was "willing to travel to Addis Ababa in the near future, sometime early next week hopefully in order to engage in a discussion with the (Ethiopian) prime minister and hopefully with Riek Machar.”

"It is safe to say that President Kiir was very open... to take forceful steps in order to end the violence and implement the cessation of hostilities agreement and to begin to engage with respect to a transitional government,” Kerry said.

Kerry said that Machar had already agreed to such a meeting, but that he would be holding further telephone talks with him later Friday to set up the talks.

Kerry’s unannounced visit has been the most determined push yet for an end to the war, which has seen the world’s youngest nation collapse amid a brutal cycle of war crimes including widespread ethnic massacres, rape and child soldier recruitment.

Threat of sanctions

Last month hundreds of people were massacred by rebels in the northern oil-hub of Bentiu – including in churches, mosques and hospitals – while a pro-government mob shot dead dozens of unarmed civilians sheltering in a UN base in the town of Bor.

Thousands of people have already been killed – and possibly tens of thousands – with at least 1.2 million people forced to flee their homes. Tens of thousands are living in appalling conditions in overstretched UN bases.

Aid agencies are also warning that South Sudan is on the brink of Africa’s worst famine since the 1980’s, with the United Nations demanding at least a one-month-long truce so that crops can be planted and food stocks boosted.

The United States has also been under pressure to intervene, having been a key backer of South Sudan’s push for secession from Khartoum and having poured in billions of dollars in aid to the country since it split from Sudan in 2011.

Speaking before his meeting with Kiir, Kerry signalled that Washington had lost patience, saying on Thursday that he was "frankly disappointed” by the conduct of both Kiir and Machar, whose bitter rivalry has sparked a wave of ethnic killings between their respective Dinka and Nuer tribes.

President Barack Obama signed a decree last month authorising punitive sanctions, including the seizure of assets and visa bans, against anyone in South Sudan deemed to be threatening peace efforts.

Although stop-start peace talks have been underway in Ethiopia since the start of the year, no progress has been made and a ceasefire signed in late January was almost immediately violated by both sides.

On Wednesday the UN High Commissioner for Human Rights, Navi Pillay, visited South Sudan and also issued a bleak assessment of the situation, saying she was "appalled by the apparent lack of concern about the risk of famine displayed by both leaders.”

The conflict started on December 15, with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting the president had attempted to carry out a bloody purge of his rivals.