Welcoming the commitment from President Salva ‘s government, they urged rebel leader Riek Machar to do likewise, as fighting continued.
Mr Machar however told BBC News that conditions for a truce were not yet in place. Although, he confirmed that two of his allies had been freed from custody, he called for the other nine to be released too.
The release of the 11 politicians, accused of plotting a coup, has been a key rebel condition for any negotiations.
Recent fighting left at least 1,000 people dead, with fierce new battles reported in the town of Malakal, in oil-rich Upper Nile State.
More than 121,600 people have fled their homes in the world’s newest state, with about 63,000 seeking refuge at UN compounds across the country, according to a statement by the United Nations.
There has been no confirmation from President Kiir’s office that he has agreed to end the hostilities in his power struggle with Mr Machar, his former vice-president, where members of Mr Kiir’s Dinka ethnic group and Mr Machar’s Nuer community have both been targeted in the violence.
East African regional leaders, who make up an eight-member bloc known as IGAD, held talks in the Kenyan capital Nairobi a day after the leaders of Kenya and Ethiopia met Mr Kiir in South Sudan’s capital, Juba.
They said they would not accept a violent overthrow of the government in South Sudan and called on the government and rebels to meet for talks within four days.
President Kiir did not attend the talks in Nairobi nor did any representative of Mr Machar.
After meeting Mr Kiir on Friday morning, US envoy Donald Booth said: “He confirmed he is moving forward to arrange a cessation of hostilities throughout the country.”
The US diplomat was also quoted by Reuters News Agency as saying Mr Kiir had agreed to release eight out of 11 politicians detained over the alleged coup plot.
“We were very encouraged to hear the president reiterate that with the exception of three… officials who have been detained… the others will be released very shortly,” Mr Booth said, according to Reuters.
Speaking to BBC World Service by satellite phone “from the bush”, Mr Machar said he was ready for talks but any ceasefire had to be negotiated by delegations from the two sides, with a mechanism agreed to monitor it.
Saying that he had the allegiance of all rebel forces in South Sudan, he called for the release of all 11 detainees.
Violence has continued through the week with conflicting reports on Friday about the situation in Malakal, capital of Upper Nile State, where some 12,000 people have been sheltering at a UN base.