Islamist leader Hasan al-Turabi said the government of President Umar Hasan al-Bashir, which seized power in a 1989 military coup, needed to move aside to let a broad-based transitional administration begin implementing any peace deal.
Failure to do so could undermine an agreement.
Al-Turabi had been the Islamist ideologue behind al-Bashir's government before he was detained in 2001 after a power struggle. He was released from house arrest in October.
"Unfortunately external pressure from the United States is just pressure," Turabi said in a telephone interview. "If a peace deal is signed under blind pressure like that there would be very little guarantee," the former ally of al-Bashir said.
"As long as you have a dictatorship in force, you never trust that they will remain loyal to their commitment. Tomorrow they may just ignore it," he said.
Talks to resume
Khartoum and the rebel Sudan People's Liberation Army, from the Christian or animist south, are due to resume peace talks in Kenya on Tuesday to end the civil war, which has killed about two million people.
The United States, which lists Sudan as a "state sponsor of terror", has played a major role in pushing the parties to peace. Washington has said it will consider removing Sudan from the list if a deal is reached.
The sides have already agreed on some major issues, including allowing the south a referendum on self-determination after a six-year interim period following the signing of a deal.
Khartoum and rebel leaders signed
wealth-sharing deal in 2003
They have also agreed on security and wealth-sharing arrangements in the interim period. Al-Turabi complained the agreements had yet to be freely published in Sudan, further isolating opposition parties from the peace process.
But what he had read in the press indicated the deal was weighing in favour of secession, even though that would not be in the economic interests of the south, which he said would be a landlocked and underdeveloped state.
Oil and foreign policy
In the 1990s, when Sudan hosted al-Qaida leader Usama bin Ladin, al-Turabi was seen as the driving force behind Khartoum's promotion of "militant groups".
"There are some forces in the United States who think that if the south can somehow mitigate the Islamism and Arabism of northern Sudan then it should remain united in the country," al-Turabi said.
"Otherwise it should secede and set itself up as a barrier against the expansion of these cultural effects into Africa," he said. Sudan's oil and the need to secure a foreign policy success also explained the US interest in Sudan, al-Turabi said.
"There are some forces in the United States who think that if the south can somehow mitigate the Islamism and Arabism of northern Sudan then it should remain united in the country"
Opposition leader, Sudan
Khartoum and the SPLM told Washington last year they would reach a deal by the end of 2003. The conclusion of a deal had been held up by government efforts to crush a one-year-old rebellion in the western Darfur region, al-Turabi said.
Al-Turabi blamed the Darfur uprising on the concentration of state power in Khartoum by a government which had ignored the Sudanese constitution's provisions for decentralisation.
"All the Sudan now is trying to pull away for more autonomy and more decentralisation and deconcentration of wealth and power," he said.
Sudanese opposition parties had agreed on the need for a broad-based transitional government picked by consensus between political parties to steer Sudan at the start of the interim period after a deal with the SPLM, al-Turabi said.
"It should be a caretaker and not stay in power for six years, but a short term (government) and prepare for elections," he said.
For many southerners, al-Turabi is a symbol of Khartoum's attempt to impose Islam and the Arabic language in the south.
Al-Turabi said he hoped animists in the south would adopt either Christianity or Islam, but he did not care which.
"If someone converts this way or that way, I don't mind myself. I don't care actually," he said. But: "I prefer a Christian to someone who is a pagan."