Feuding rebel leaders briefly walked out of a meeting with a top U.S. envoy Tuesday, giving him an object lesson in the challenges he faces as he tries to shore up peace efforts in Sudan.

The U.S. State Department's No. 2 official Robert Zoellick had called the meeting to try to unite the factions within the Sudan Liberation Movement, the largest rebel group in Sudan's western Darfur region. The split within the rebel movement has undermined peace talks with the government and sparked more fighting in Darfur.

At a meeting last week in Darfur, a faction led by Minni Minnawi claimed control of the movement. But Minnawi's rival Abdel Wahid Nur boycotted that meeting.

Nur was the first to arrive Tuesday for talks with Zoellick. He was later joined in a conference room at a luxury hotel by a deputy sent by Minnawi. When Zoellick arrived, both Darfur factions walked out.

Saif Haroun, a spokesman for Minnawi, said the Nur faction wanted equal footing with the Minnawi faction during the talks with Zoellick.

"They can join the talks as members of the SLM, but they cannot claim to be leaders at all," Haroun said.

Zoellick's aides persuaded both factions to return to the conference room, but it was not clear under what conditions.

"We cannot tell them who should be their leaders, we cannot tell them who should be their representatives — but we can tell them they better get their act together," the African Union envoy to Sudan, Baba Gana Kingibe, told The Associated Press after taking part in the meeting with Zoellick, the rebels and other diplomats.

Zoellick said after the meeting that he and diplomats representing the African Union, Canada, the European Union, Netherlands, Norway and the United Nations pressed the rebel factions to adopt a common negotiating position for ongoing peace talks with the Sudanese government.

The rebels were also urged to "respect the cease-fire, work with (the African Union peacekeeping mission) and protect humanitarian workers," Zoellick said before flying to Sudan to meet with government officials.

Nur told The Associated Press that his faction was willing to work with his rivals toward a common negotiating position for the next round of peace talks, scheduled for later this month in the Nigerian capital.

Speaking to reporters on the flight to Kenya from Washington Monday, Zoellick said violence had increased in Darfur over the course of the past few weeks, and that that may be related to power struggles within the Sudan Liberation Movement, also known as the Sudan Liberation Army.

"That violence risks unraveling the very fragile situation in Darfur," Zoellick said. "We have got to regain the moment by getting the people to respect the cease-fire, come up with a coherent negotiating position and when negotiations resume on Nov. 20 to get the rebels and the government of national unity to make more progress on the peace accord because fighting and killing will not provide a solution."

The African Union and Chad have been co-mediating peace talks to end the Darfur conflict, with the last session ending on Oct. 20 in the Nigerian capital, Abuja. The parleys are set to resume later this month.

After decades of clashes over land and water in Darfur that often pitted the region's ethnic Arab tribes against its ethnic African tribes, conflict erupted on a wider scale in February 2003. Then, the Sudan Liberation Movement and the other major rebel group, Justice and Equality Movement, took up arms against the Sudanese government amid accusations of repression and unfair distribution of wealth.

The central government is accused of responding by unleashing ethnic Arab tribal militias known as Janjaweed to murder and rape civilians and lay waste to villages. The Sudanese government denies backing the Janjaweed.