The head of one of Darfur's main rebel groups said he is willing to call a cease-fire if Sudan's government stops attacks on civilians in the war-torn region and agrees to renegotiate a peace deal — but warned of a new offensive if it fails to do so.

Khalil Ibrahim heads the Justice and Equality Movement, which along with most other rebel groups has refused to sign onto the Darfur Peace Accord. The United Nations is trying to get the government and rebels into negotiations to rework the deal.

Since the accord was signed last May, violence has only increased in the western region of Sudan, with fighting between government and rebels and continued attacks by the pro-government Arab janjaweed militia, which is accused of widespread atrocities against Darfur civilians.

The United Nations warned this week that a large number of militiamen — apparently janjaweed — were massing near El-Geneina, the capital of West Darfur, raising fears of more violence. U.N. spokeswoman Radhia Achouri said the reason for the gathering was not known and would not say how many militiamen were there. The West Darfur governor denied the report.

"If Khartoum doesn't reopen peace negotiations and doesn't immediately cease janjaweed attacks, we will have no choice but a large-scale offensive," Ibrahim said in an interview with the Associated Press.

"We will agree to an immediate cease-fire if there is a framework for new peace talks. We are waiting for the government's response," Ibrahim said. "Either we get the opportunity to negotiate a real treaty, an acceptable peace that we can sell to our people, or we will proceed."

Ibrahim, whose JEM leads a coalition of rebel groups, spoke outside the town of Abeche in eastern Chad, close to the border with Sudan and Darfur. Around 40 of his fighters were with him, sitting in the shade of trees in a dried-up riverbed.

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Ibrahim made clear his forces were ready to keep fighting. He dismissed reports that Sudan was using a flood of new oil income to build up its armed forces, saying the rebels capture many government weapons.

"This is all provided by the Sudanese government," he said, pointing to five pickup trucks that looked like the ones used by the Sudanese army, laden with rocket-propelled grenades and heavy machine guns.

Ibrahim spoke last week after coming out of a meeting with a senior Chadian general. Sudan accuses Chad of backing the Darfur rebels. Ibrahim insisted JEM — whose leadership is a mix of former Islamists and former communists — gets no foreign support, but acknowledged that Chad gave free passage to his men.

"Our men our fighting for a cause, for our land and for our people," he said. "Every time the (Sudanese) army comes out in the open, we defeat them."

The size of the rebel forces has long been "murky" and in constant flux, but the JEM is thought to number only several hundred fighters, said Stephen Morrison, the director of the Africa Program at the Washington-based Center for Strategic and International Studies. The faction of the Sudan Liberation Army that continues fighting is larger, with about a thousand.

But in more than three years of fighting, they have proved resilient and have the advantage of popular support, an abundant supply of weapons in the region and knowledge of the terrain, he said. They also benefit from the relative weakness of the Sudanese military. The janjaweed are "more of a marauding force ... They're better at raping and pillaging than engaging in standing battles," Morrison said.

Ibrahim claimed to have far more than several hundred fighters, but would not specify how many.

He cited a major battle last September in Umm Sidir in northern Darfur, where rebels crushed a 4,000-strong government force — capturing over 100 vehicles, heavy armament and a Sudanese army general, along with hundreds of soldiers.

"Do you really think there's only a few hundred of us?" he said. JEM troops also downed two army helicopters in December during a raid against the Abu Jabra oil fields in Kordofan, halfway between Darfur and Khartoum.

More than 200,000 people have been killed in Darfur since 2003, when ethnic Africans launched their rebellion against the Arab-led government. More than 2.5 million people have fled their homes, and the conflict has begun to spill over into neighboring Chad.

The May peace deal was rejected by most Darfur refugees and rebels, who said it gave no guarantees the government would stop fighting, did not provide compensation and did not directly provide for a U.N. peacekeeping force for Darfur.

Only one rebel leader, Minni Minawi of the Sudan Liberation Army accepted. He has since received a top government post but has lost much support in Darfur, and a faction broke away to continue fighting.

Ibrahim said he felt cheated by the May deal, which he said was imposed by American and European envoys, who "wanted to tell their people they'd solved the problem in Darfur, when in fact they only made it worse."

Ibrahim urged the U.N. to send peacekeepers to Darfur despite Sudan's rejection of a plan to send some 22,000 U.N. troops to replace the overwhelmed African Union force there.