By Opheera McDoom

El-FASHER, Sudan, Dec 1 (Reuters) - Failure to deploy a joint U.N.-African Union peacekeeping mission could push Sudan's Darfur region back into a spiral of violence, the military head of the mission said on Saturday.

U.N. peacekeeping chief Jean-Marie Guehenno said last week Khartoum had demanded such stringent restrictions on the 26,000-strong force that its deployment was in doubt.

"If people are disappointed this could be expressed in many ways and one of them could be in a violent way and then ... we will go back to square one," force commander Martin Luther Agwai said.

On the possibility the force would not deploy, he said: "As the man on the receiving end I'm disturbed and concerned."

Khartoum agreed to the joint force after months of threats, negotiations and talks but extracted heavy concessions from the international community for that agreement.

The force will replace a struggling AU mission of almost 7,000 personnel which has failed to stop the conflict which the U.N. estimates has killed 200,000 in a terrible humanitarian crisis and forced 2.5 million from their homes.

Sudan has yet to provide land in el-Geneina and Zalengei towns in West Darfur, U.N. spokesman Ali Hamato said. With the joint mission due to take over by the end of the year Agwai said the mission was months behind schedule.

He said the best case scenario was to have 6,500 troops on the ground and maybe an additional 3,000 police by the takeover.

"If you look at that 9,000 are asked to do the job of the barest minimum of 26,000, you will know how Herculean that task is," he said.

Agwai said even the 26,000-strong force agreed was the bare minimum needed to keep peace in the region the size of France.

He expressed surprise Western nations had not offered the 24 helicopters including six attack helicopters, vital for the force to defend itself and the people of Darfur.

"I thought with all the suffering...that the people would have lined up," he said.

Agwai said there had been numerous reports of ceasefire violations since the government declared a unilateral truce in October. Because these violations were in areas controlled by rebels who reject a May 2006 peace deal, the ceasefire commission could not investigate.

"It has become next to impossible to verify some of these claims," he said. "I will say there seems to have a heightening (in violence) and I don't think they are unconnected to what is happening in Chad," he said.

Violence in Chad's east which borders Darfur has escalated in recent weeks and the army says hundreds have been killed in clashes with rebels. (Editing by Robert Woodward)

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