President Bush talked by telephone Tuesday with leaders in Sudan (search), pushing for progress in peace talks to end a 21-year civil war in southern Sudan.

Bush urged President Omar el-Bashir (search) to push ahead to an agreement on the north-south conflict. The American president also stressed the need to resolve the separate conflict in Darfur (search) region, which has pitted western rebels against Arab militias and government troops and caught up tens of thousands of civilians in the conflict.

Bush also talked with U.S.-educated southern rebel leader John Garang about the conflict between his Sudan People's Liberation Army (search) and the government over control of the southern war could serve as a model for solving the Darfur conflict. The United States and other countries blame government-affiliated Arab militias for violence that has killed an estimated 70,000 people in Darfur and forced more than 1.2 million from their homes.

The United Nations calls it the world's worst humanitarian crisis. U.N. Secretary-General Kofi Annan has spoken of evidence of widespread war crimes.

Garang and Sudan's vice president, Ali Osman Taha (search), are to meet Dec. 11 to work out final details of how to implement a comprehensive peace agreement that John Danforth, the U.S. ambassador to the United Nations, helped broker when he was the Bush administration's special representative to Sudan.

The government and rebels have agreed to form a 24,000-member national army, with half the troops from each side. They are at odds over a rebel demand to maintain a separate army as a security guarantee during a six-year transition period. They also disagree on how and when to disarm other groups in southern Sudan and how to incorporate members into rebel and government forces .

The conflict broke out in 1983 as black African rebels from the mainly animist and Christian southern Sudan took up arms against the predominantly ethnic Arab and Muslim north. Most of the 2 million casualties have come from war-induced famine. The insurgents say they are fighting for better treatment and for southerners to have the right to choose whether to remain part of Sudan.

The U.N. Security Council (search) opens a special session on Sudan on Thursday in Nairobi, Kenya.

"They will be focusing on getting the talks for the north-south peace accords moving toward a resolution," White House press secretary Scott McClellan said. "Among other things, it's imperative that the government of Sudan and the Sudan People's Liberation Movement (search) conclude the comprehensive peace agreement when talks resume later this month."