South Sudan's government refused to sign a peace deal with rebels Monday despite the threat of international sanctions, but will return to finalise an agreement within 15 days, mediators said.
Rebel chief Riek Machar however said he had signed a deal, and called on President Salva Kiir to join.
"It was an opportunity for us to end the war," Machar said.
"We call on President Kiir to reconsider his position so that they can sign and we can go forward," he added.
The secretary-general of the ruling party Pagan Amum also signed the deal, but mediators said he was not representing the government.
Kiir — who watched the signing and briefly shook hands with Machar — had warned from the start of talks it would not be possible to sign a credible peace deal because rebel forces have split.
Powerful rebel general Peter Gadet and other key commanders last week accused Machar of seeking power for himself, and said they would not recognise any deal agreed.
Kiir, who earlier said a "peace that cannot be sustained cannot be signed," left without comment after the meeting Monday.
Washington, which helped mediate the talks, expressed regret the president had not signed and urged him to do so.
"We call on the government to sign the agreement within the 15-day period it requested for consultations," State Department spokesman John Kirby said.
The international community had threatened possible sanctions if a deal was not reached by the end of the day, and it was not clear if the result of talks on Monday would ward off any repercussions.
– Tens of thousands dead –
Mediators however said progress had been made.
"This is a great day in the movement of the peace process in South Sudan," chief mediator Seyoum Mesfin said, but added that "the signing ceremony is not complete without the signing of the government."
Rival leaders have been under intense diplomatic pressure to end 20 months of a brutal civil war in which tens of thousands of people have been killed.
The government has "certain reservations" and will return after consultations, Mesfin said.
"In the next 15 days, the president will come back to Addis Ababa and finalise the peace agreement," he added.
Key issues of disagreement include a power-sharing proposal between the government and rebels, which could see Machar return as vice-president.
At least seven ceasefires have already been agreed and then shattered within days — if not hours — in the world's newest country, which broke away from Sudan in 2011.
For his part, Machar insisted the deal he signed on Monday was "meaningful."
African Union chief Nkosazana Dlamini-Zuma has warned the failure to strike an agreement "will have far reaching consequences for South Sudan, the region and the continent as a whole."
Monday's ceremony ended 10 days of talks mediated by the regional eight-nation bloc IGAD, the Intergovernmental Authority on Development, as well as the United Nations, AU, China and the "troika" of Britain, Norway and the United States.
– Ethnic massacres and rape –
Regional leaders who have appeared at the talks include host, Ethiopian Prime Minister Hailemariam Desalegn, and Ugandan President Yoweri Museveni, who has sent troops into South Sudan to back Kiir. The presidents of Djibouti, Kenya and Sudan were also present.
South Sudan's civil war erupted in December 2013 when Kiir accused his former deputy Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.
In a signal of the dire conditions on the ground, the number sheltering inside UN peacekeeper bases has risen by a third in just over a month to almost 200,000 civilians, the UN mission said Monday.
Marked by widespread atrocities on both sides, 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.
More than 70 percent of the country's 12 million people need aid, while 2.2 million people have fled their homes, the UN says, with areas on the brink of famine.
