He 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.
The first UN reinforcements have arrived to help control civilians seeking shelter at UN compounds.
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 1,000 people have died in the fighting.
President Salva Kiir is engaged in a deadly power struggle with Mr Machar, his former vice-president, since accusing him of a coup attempt. Members of Mr Kiir’s Dinka ethnic group and Mr Machar’s Nuer community have both been targeted.
The government has freed two of Mr Machar’s allies from detention, partially meeting a rebel demand for the release of 11 coup plotters.
US envoy Donald Booth said in Juba he hoped the freed detainees would “participate in a constructive manner in the efforts to bring about peace… and resolve the political issues that sparked this conflict”.
Mr Machar spoke to the BBC World Service by satellite phone “from the bush”, as he put it.
He said he had a negotiating team ready but any ceasefire had to be serious, credible and properly monitored.
“So until mechanisms for monitoring are established, when one says there is a unilateral ceasefire, there is no way that the other person would be confident that this is a commitment,” he said.
He called for the release of all 11 detainees, a key rebel condition for any negotiations.
Mr Machar said the rebels controlled the whole of Unity State, three-quarters of Upper Nile State and all of Jonglei State except the state capital Bor.
He said he had spoken to the two detainees freed in Juba, whom he named as his executive director, Deng Deng Akon, and former Higher Education Minister Peter Adwok.
The government tweeted to say it had agreed “in principle to a ceasefire to begin immediately”. Presidential spokesman Ateny Wek told regional broadcaster Radio Tamazuj: “It is not a unilateral offer, but it is a conditional offer to be accepted by the other party.”
In Nairobi, South Sudanese Information Minister Michael Makuei Lueth said the government had agreed to suspend a planned offensive to recapture Bentiu, the capital of Unity State.
“We are not moving on Bentiu as long as the rebel forces abide by the ceasefire,” he was quoted as saying by the Associated Press.
East African regional leaders meeting in the Kenyan capital Nairobi said they would not accept a violent overthrow of the government and called on the warring sides to meet for talks within four days.
The first UN reinforcements have arrived since the UN Security Council voted to almost double the number of peacekeepers to 12,500.