East African leaders met in Ethiopia on Sunday ahead of a deadline for South Sudan's warring leaders to strike a peace deal or risk international sanctions.

South Sudan's government and rebels are under intense diplomatic pressure to sign a deal by August 17 to end a 20-month civil war in which tens of thousands of people have been killed.

But South Sudanese President Salva Kiir has said he would not go to the talks and would send his deputy instead, after complaining it was not possible to strike an effective deal because rebel forces have split.

Vice-President James Wani Igga is expected to take his place, a presidential spokesman said.

The latest round of talks opened on August 6, mediated by the regional eight-nation bloc IGAD, the Intergovernmental Authority on Development, as well as the United Nations, African Union, China and the "troika" of Britain, Norway and the United States.

Diplomats have warned any failure to sign a peace deal could trigger "serious consequences" for the rival leaders.

"We expect the parties to be represented in Addis Ababa by their principals, in order to negotiate in good faith and sign an agreement," IGAD and international mediators said in a statement.

Britain's minister for Africa, Grant Shapps, warned on Friday of possible "targeted sanctions" and an arms embargo if no deal is made.

The war has been marked by widespread atrocities on both sides.

South Sudan's civil war began in December 2013 when 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.

On Sunday, Ugandan President Yoweri Museveni, who has sent troops into South Sudan to back Kiir, said in a statement that he had held closed-door meetings with Ethiopian Prime Minister Hailemariam Desalegn and Sudanese President Omar al-Bashir.

Kenyan President Uhuru Kenyatta travelled on Sunday morning to join them, Kenya's government said.

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 assistance, while 2.2 million people have fled their homes, the UN says, with areas on the brink of famine.