Speaking at an extraordinary heads of state meeting held by east African body Inter Governmental Authority on Development (IGAD), Kenyan President Uhuru Kenyatta urged Kiir and ex-vice president Riek Machar to seize "the small window of opportunity" and start peace talks.

"Let it be known that we in IGAD will not accept the unconstitutional overthrow of a duly and democratically elected government in South Sudan. Violence has never provided optimum solutions," Kenyatta said, according to a statement released by the State House. 

East African leaders are to meet Friday in Nairobi to discuss escalating violence in South Sudan, said Hilde Johnson, head of the UN mission to the troubled new country. Leaders of the Intergovernmental Authority on Development, or IGAD, are to meet today following three-way talks in Juba between South Sudanese President Salva Kiir, Kenyan President Uhuru Kenyatta and Ethiopian Prime Minister Hailemariam Desalegn.

Those "constructive" discussions centred on the cessation of hostilities that have left hundreds dead since last week, Ethiopian Foreign Affairs Minister Tedros Abodon told journalists Thursday.

The leaders also discussed the humanitarian crisis caused by the violence and possibilities for a political dialogue with Kiir's rival, former vice president Riek Machar, he said.

Ethiopia and Kenya are both founding members of IGAD, which includes Djibouti, Somalia, Uganda, Sudan since the bloc's founding in 1986, and South Sudan since that country's independence in 2011.

It is not clear how many of the countries will be represented.