The US special envoy to South Sudan said Thursday that patience has run out over the country's civil war, pressing the message that the warring parties were running out of time to strike a deal and stop fighting.

Regional mediators, backed by US President Barack Obama during his recent visit to Kenya and Ethiopia, have given South Sudan's rivals until August 17 to stop the 19-month-old civil war.

""I want to be very clear with you all: the patience of my country, of the region and of the other international partners has run out," US envoy Donald Booth told reporters in Juba.

"Too many lives have been lost, too many millions of South Sudanese have been displaced and too many are at the verge of starvation and facing homelessness. The talks can't continue without end," he said.

"This situation can't go on any longer."

Obama held talks with regional leaders earlier this week in an attempt to build African support for decisive action against the war-torn country's leaders if they reject the ultimatum to end the carnage.

South Sudan's warring leaders -- President Salva Kiir and rebel leader Riek Machar -- effectively face an ultimatum, a "final best offer," according to one senior US administration official.

A failure to strike a deal could then lead to a range of punitive measures including an arms embargo and targeted sanctions including travel bans and asset freezes.

Booth said peace delegates would meet in Addis Ababa on August 6, and that he expected them to "work this out" once and for all.

The regional eight-nation Intergovernmental Authority on Development (IGAD) bloc has been trying to mediate a peace deal since the start of the conflict, which began when Kiir accused Machar, a former vice president, of attempting a coup.

But the IGAD initiative has so far proved to be a failure. The peace process has seen scores of South Sudanese delegates hosted in luxury hotels in Addis Ababa and successive ceasefire deals quickly fall apart -- sometimes within hours.

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.

The conflict has left tens of thousands dead and has been marked by widespread atrocities on both sides.