Obama says time for 'breakthrough' in South Sudan peace

US President Barack Obama said on Monday the situation in civil war-torn South Sudan was deteriorating but that it was now time for a "breakthrough" in peace efforts.

"Unfortunately, the situation continues to deteriorate. The humanitarian situation is worsening. The possibilities of renewed conflict in a region that has been torn by conflict for so long and has resulted in so many deaths needs addressing," Obama said in the Ethiopian capital Addis Ababa.

"We don't have a lot of time. The conditions on the ground are getting much much worse. The goal is to make sure the US and IGAD (a regional bloc leading the peace process) are aligned on a strategy going into this endgame on peace talks," he said.

"Now is time for a breakthrough."

Obama, making the first-ever visit to Ethiopia by a sitting US president, is to hold talks with regional leaders on the civil war in South Sudan in an attempt to build African support for decisive action against the war-torn country's leaders if they reject an ultimatum to end the carnage by 17 August.

South Sudan's warring leaders - President Salva Kiir and rebel leader Riek Machar, who will not be at the meeting - effectively face an ultimatum, a "final best offer", according to one senior administration official.

South Sudan, midwifed into existence by US cash and support in 2011, has faltered badly in its infancy and the Obama administration has been accused of abandoning the fragile nation.

Recent celebrations for the country's fourth anniversary were marred by reports of violence as political efforts have failed to end the crisis.