South Sudan's rival forces began peace talks Thursday, as international pressure mounts ahead of an August 17 deadline to strike a deal to end 19 months of civil war.
The conflict has left tens of thousands dead and has been marked by widespread atrocities on both sides.
"We have now reached a critical juncture whereby participants of this phase will make decisions that may impact the destiny of the people of South Sudan," chief mediator Seyoum Mesfin said at an opening ceremony.
Delegates met in the Ethiopia capital Addis Ababa, under mediation from the regional eight-nation bloc IGAD, the Intergovernmental Authority on Development.
South Sudan's civil war began in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.
"We have not been able to count the number of dead," Seyoum said.
Diplomats have said repeatedly that patience has run out over the country's civil war.
"You have had enough time to consult, since the proposals are the very same issues that you have been discussing for the last 19 months," Seyoum said.
But the mood was far from friendly, with chief rebel negotiator Taban Deng saying the government in Juba had "lost all legitimacy" while his government counterpart, Nhial Deng Nhial, was dismissive of proposals that he said put Machar on a level footing with Kiir.
Regional mediators, backed by US President Barack Obama during his recent visit to Kenya and Ethiopia, have given Kiir and Machar until August 17 to halt the civil war.
Kiir and Machar, who are expected to appear later in the process, effectively face an ultimatum, a "final best offer," according to one senior US administration official.
Failure to strike a deal could lead to a range of punitive measures including an arms embargo and targeted sanctions, including travel bans and asset freezes.
After the collapse of the last round of talks, mediators expanded involvement to create an "IGAD-plus" group.
It now also includes the United Nations, five more African Union nations from across the continent -- Algeria, Chad, Nigeria, Rwanda and South Africa -- as well as China and the "Troika": Britain, Norway and the US.
Norwegian diplomat Jens-Peter Kjemprud, speaking for the Troika, warned the parties of the need to strike a deal.
"You have the opportunity to resolve outstanding issues before August 17," he said. "If not, the international community will look at new ways to resolve the conflict."
During previous peace talks held in luxury Ethiopia hotels, Kiir, Machar and their entourages have run up millions of dollars in expenses while failing to sign a single lasting agreement.
At least seven ceasefires have been agreed and then broken within days, if not hours.
At home, South Sudan's bishops have called on leaders to put the fate of the country before their pursuit of power and settle a deal.
"There is no moral justification for this senseless war," the influential Council of Churches, a coalition including all the key Protestant and Roman Catholic bishops, said in statement.
"It is unacceptable that people continue to kill and be killed while leaders argue over power, positions and percentages."