KHARTOUM, Sudan — The government of South Sudan refused on Saturday to release all 11 senior politicians who were detained on allegations of plotting a coup as a condition for a truce with rebel forces.

"It is not accepted," Michael Makuei Lueth, South Sudan's information minister, said in a phone interview. "That is a condition, and we said we will enter negotiations with no preconditions."

The 11 were arrested two weeks ago after President Salva Kiir said they had joined the former South Sudanese vice president, Riek Machar, in plotting to overthrow the government. The arrests inflamed violence across the country, which is now believed to have killed thousands of people. Mr. Machar, who denied orchestrating any coup attempt, fled the capital, Juba, and his whereabouts have not been disclosed.

On Friday, East African leaders trying to end the crisis announced that they had made progress on a peace deal. The South Sudanese government said it would release eight of the prisoners, and later freed two of them: Deng Deng Akon, the former executive director for Mr. Machar's office, and Peter Adwok, the former higher education minister.

Mr. Machar remained skeptical about the proposed cease-fire, and told the BBC, "All the detainees must be released before we talk substantial issues."

But Ateny Wek Ateny, the presidential spokesman, said Saturday that the government had no plans to release three of those detained, and would continue investigating the other eight men to see if they were part of a coup attempt.

The three it refuses to release are Kosti Manibe, the former finance minister; Deng Alor, the former cabinet affairs minister; and Pagan Amum, the former secretary general of the governing party, the Sudan People's Liberation Movement. All three are senior members of the party.

"They are being held on charges of corruption," Mr. Ateny said in an interview.

What began as a political struggle in the party soon took on an ethnic dimension as fighting broke out between the two largest ethnic groups in South Sudan, the Dinka and the Nuer. Mr. Kiir is a Dinka, and Mr. Machar a Nuer.

The conflict has set off a humanitarian crisis, with tens of thousands seeking refuge at United Nations compounds throughout the country. The United Nations has called for $166 million in humanitarian aid.

The East African leaders who met Friday in Nairobi, Kenya, to discuss the crisis issued a communiqué saying they "welcomed the commitment by the government of the Republic of South Sudan to an immediate cessation of hostilities," and called upon Mr. Machar to follow suit. But Mr. Machar was noncommittal.

"Until mechanisms for monitoring are established, when one says there is a unilateral cease-fire, there is no way that the other person would be confident that this is a commitment," he told the BBC.

Rebel forces allied with Mr. Machar lost control of the strategic towns of Bor and Malakal last week. But Bentiu, the capital of the oil-producing Unity State, remains under their control.

"We will not attack first but defend ourselves if attacked," Mr. Lueth said.

Referring to Bentiu, he added, "Ultimately, however, we will not allow them to remain in control of the city."