United Nations chief Ban Ki-moon is expressing disappointment at an agreement between South Sudan's warring parties, billed as a key step toward a peace deal.

In a statement released through his spokesman, the U.N. secretary-general said he regrets that neither President Salva Kiir nor rebel chief Riek Machar compromised to reach a mutually-acceptable power-sharing formula.

The statement Tuesday said "no sustainable peace will be found in South Sudan unless its leaders place the interests of the civilian population above their own."

The agreement, signed Sunday by Kiir and Machar, said the South Sudanese government and rebel forces must finalize a peace deal by March 5, but gave few details on how that would be achieved.

The two sides have been at war for more than year, leaving more than 10,000 people dead and driving about 1.5 million people from their homes.

The fighting stemmed in part from the political rivalry between Kiir and Machar, who the president fired as his deputy in July 2013.

The government and rebels have previously signed at least three peace deals which were quickly broken.

The East African bloc IGAD has been mediating peace talks in Ethiopia. IGAD mediator Seyoum Mesfin said anyone who breaks this latest agreement will be reported to the United Nations Security Council and the African Union.

Both organizations have threatened sanctions against those undermining peace in South Sudan, the world's newest country.