The rebel leader in South Sudan says there must be proper negotiations before a truce can be agreed with the government.
Riek Machar was speaking after the government said it had agreed to an immediate end to fighting, in a declaration welcomed by East African leaders in Nairobi.
Eight regional leaders held talks in Nairobi on Friday after the leaders of Kenya and Ethiopia met President Salva Kiir in South Sudan's capital, Juba, on Thursday.
The president says his government has agreed to an immediate end to the fighting, which began about two weeks ago.
President Kiir is engaged in a deadly power struggle with Mr Machar, his former vice-president, since accusing him of a coup attempt.
Mr Machar says his forces still control much of the north of the country. He told the BBC he has a negotiating team ready, but any ceasefire has to be serious, credible and properly monitored.
About 63,000 people have turned to the UN for help, among more than 121,600 who have fled their homes in the world's newest state. At least 1000 people have died in the fighting.
The fighting is affecting oil production, which accounts for 98% of government revenue.
The UN Security Council has voted to almost double the number of UN peacekeepers to 12,500.