Two years of Darfur peace talks characterized by setbacks and frustration ended with a final flurry of international diplomacy and a signing ceremony Friday.

Now the hard part: ensuring pledges to stop the fighting and being the rebuilding translate to an end to suffering in western Sudan. The key may be a robust U.N. peacekeeping force, which Sudan's government indicates it is now willing to accept.

While the main Darfur rebel group signed the accord backed by the African Union, United States, Britain, the European Union and the Arab League, two others rejected it, saying it did not go far enough to meet their demands for security and power sharing guarantees and compensation for war victims. Optimism was muted the two groups' absence and by a history of failure to live up to agreements.

Members of the fractious rebel camp are united in accusing the central government of neglecting their impoverished region, but divided because of leadership rivalries and differing approaches.

CountryWatch: Sudan

The peace deal calls for a cease-fire; disarmament of so-called Janjaweed militias linked to the government and accused of some of the war's worst atrocities; the integration of thousands of rebel fighters into Sudan's armed forces; and a protection force for civilians in the immediate aftermath of the war. Political provisions include guarantees rebel factions will have the majority in Darfur's three state legislatures, but the rebels did not get the national vice presidency they had sought.

U.S. Deputy Secretary of State Robert Zoellick indicated the two smaller rebel groups could be bypassed, and his assessment was bolstered when one of the two split Friday, with dissenters criticizing their leader for not embracing the proposed treaty. Zoellick said implementing the agreement would be a challenge, but said he was looking ahead next to organizing a U.N. peacekeeping force for Darfur.

The Sudanese government initially rejected calls for U.N. peacekeepers to replace the thousands of African Union peacekeepers in Darfur now, but indicated Friday it would yield once the peace treaty was signed.

Observers broke into applause and whoops of joy as the parties signed the last page and then proceeded to initial each of the 85 pages of a document written by the African Union and then revised by U.S., British and other envoys to meet rebel concerns. The hall in a presidential villa was filled with traditional leaders in white turbans, fighters in camouflage turbans, diplomats and journalists.

"Unless the right spirit, unless the right attitude and right disposition is there, this document isn't worth the paper it is signed on," Nigerian President Olusegun Obasanjo, a key figure in peacemaking efforts across Africa and host of the protracted Darfur talks, said at the ceremony. "Those who don't sign, we will continue to appeal to them. The window of opportunity must not be allowed to close."

U.N. High Commissioner for Human Rights Louise Arbour, just returned from a tour of Darfur, said in Khartoum Friday the peace deal "will open up space for improvement," but first must be implemented. She added she hoped it would encourage donors to come back and help the desperate people of Darfur.

The largest rebel group, Minni Minnawi's faction of the Sudan Liberation Movement signed. The SLM faction led by his rival, Abdel Wahid Nur, and the smaller Justice and Equality Movement rejected the accord, expressing concerns security and compensation for war victims had not been guaranteed and because it called for a top presidential adviser from Darfur instead of a vice president.

Nur met with Obasanjo for hours Friday, delaying the signing ceremony, and then briefly went into the hall where the accord was to be signed.

He left telling reporters the proposed accord was "a big disaster" because he believed it did not go far enough to guarantee disarmament of the Janjaweed. Nigerian security tried to stop Nur from speaking to reporters, then barred reporters who had followed him out from returning to witness the signing.

Sudan's government agreed days ago to an initial proposal drafted by African Union mediators and has been flexible as U.S. and British officials fine-tuned it to address rebel concerns.

Deadlines have been extended twice since Sunday and Thursday's session went five hours beyond the midnight time limit.

At least 180,000 people have been killed and more than 2 million forced to flee their homes in what the United Nations has called one of the world's worst humanitarian crisis. The Darfur conflict, which erupted in February 2003, also has spilled into Chad and Central African Republic. The violence threatens to escalate: Usama bin Laden last week urged his followers to go to Sudan to fight a proposed U.N. presence.

Aid groups say the security situation must be quickly addressed, in part to allow them to do their work. Oxfam called Friday for bolstering the African Union force already in Darfur, rather than awaiting U.N. peacekeepers who could take months to arrive.

"The deteriorating situation in Darfur must be addressed urgently, and not put off until if or when a U.N. force may be in place," said Paul Smith-Lomas, who directs the Darfur operations of the British aid group Oxfam.