South Sudan's warring leaders failed to reach a deal to end more than a year of civil war, mediators said Friday, with the latest collapse in peace talks paving the way for possible sanctions.

Ethiopia's prime minister said South Sudan's President Salva Kiir and rebel leader Riek Machar missed a deadline to reach a peace agreement by midnight Thursday, and that further talks on Friday "did not produce the necessary breakthrough."

The foreign ministers of Sudan, Egypt and Ethiopia said early on Friday they had reached the basis of an agreement on the sharing of Nile waters and Ethiopia's Grand Renaissance Dam.

"A full agreement has been reached between our three countries on the principles of the use of the eastern Nile Basin and the Ethiopian Renaissance Dam," Sudanese Foreign Minister Ali Karti told reporters early on Friday morning.

South Sudan's warring rivals headed for the latest round of peace talks Friday, with government negotiators saying they were "optimistic" of a deal despite fighting continuing.

"We are going there with high hopes," Information Minister Michael Makuei and government delegation chief told reporters as they left for the Ethiopian capital Addis Ababa. Seven previous ceasefire deals have failed.

South Sudan President Salva Kiir and rebel leader Riek Machar late Sunday signed a new agreement to end more than 13 months of fighting in a civil war that has left tens of thousands dead.

"Complete cessation of hostilities in South Sudan is expected as of this morning (Monday)," Seyoum Mesfin, a negotiator from the regional IGAD bloc, told reporters in the Ethiopian capital Addis Ababa where the ceasefire deal was signed.

After 13 months of fighting and six failed ceasefires, diplomats are being forced to accept that any deal to end the war in South Sudan will, at best, result in a return to the status quo that precipitated the carnage in the first place.

The latest peace proposal drafted this week in Addis Ababa by regional bloc IGAD and seen by AFP leaves Salva Kiir as president and re-installs rebel leader Riek Machar as his deputy, a position he held until July 2013 when his sacking planted the seed of a war that erupted five months later.

South Sudan's president and rebel leader met Thursday for the latest round of peace talks mediated by East African leaders aimed at ending their 13-month-old civil war.

President Salva Kiir and rebel leader Riek Machar, whose rival armies continue to clash met face-to-face, met alongside presidents from the East African regional IGAD bloc in the Ethiopian capital Addis Ababa.

The Ethiopian government is stepping up oppression of independent journalists ahead of national elections due in May, a leading rights group said in a report on Thursday.

"Ethiopia's government has systematically assaulted the country's independent voices, treating the media as a threat," said Leslie Lefkow, deputy Africa director at Human Rights Watch.

An Ethiopian court has jailed three British men for between four and six years for allegedly plotting to "establish an Islamic state in Ethiopia", a report said.

Mohamad Sharif and Mohamad Ahmed, who had both been reportedly living in London, were both sentenced to six years and eight months in jail on Wednesday, after the court found them guilty of receiving military training in neighbouring Kenya.

Civil society groups in war-torn South Sudan warned Tuesday the country needs peace to hold elections, and said the government should extend its mandate rather than vote with conflict raging.

The government announced earlier this month presidential and parliamentary polls would be held in the bitterly divided nation between May and July, the first time elections would be held in South Sudan as an independent country.

East African foreign ministers met in Mogadishu on Saturday to push peace efforts in war-torn Somalia, the first time the regional bloc has met in the country for almost three decades.

Dozens of heavily armed soldiers and police patrolled the streets, where Al-Qaeda-affiliated Shebab militants regularly carry out bombings and killings.