(Recasts with Security Council statement)

By Patrick Worsnip

UNITED NATIONS, Oct 24 (Reuters) - The U.N. Security Council demanded on Wednesday that Sudan's government and Darfur rebels attend peace talks starting over the weekend and threatened action against anyone impeding the negotiations.

The council statement, which said an urgent cease-fire was the talks' first priority, came after a U.N. envoy appealed to the reluctant and disunited rebel groups to show up, saying they could still consult each other as negotiations proceed.

One of several obstacles to the talks between the Khartoum government and rebels from Darfur in western Sudan has been the rebels' inability to agree a common platform. The parties are set to meet on Saturday in the Libyan coastal town of Sirte.

Rebel leaders said on Tuesday a prominent Darfur faction chief and five smaller groups would not attend because African Union and U.N. mediators had not heeded requests for a delay to let them form a united position and agree upon a delegation.

The council called on "all parties to attend and to engage fully and constructively in the talks and, as a first step, to urgently agree and implement a cessation of hostilities to be overseen by the United Nations and African Union."

"The Council underlines its willingness to take action against any party that seeks to undermine the peace process, including by failing to respect such a cessation of hostilities or by impeding the talks, peacekeeping or humanitarian aid," the official policy statement added.

The action was not spelled out and diplomats said failure to attend the talks would not qualify for it. The statement "doesn't mean that simply non-appearance equates to impeding the peace process," said British Ambassador John Sawers.

Earlier, U.N. envoy Jan Eliasson, who will mediate in the talks along with AU envoy Salim Salim, said the "moment of truth" had arrived in the 4 1/2-year-old Darfur conflict and it was essential to get the talks going before momentum was lost.

Talking to journalists at the United Nations by video link from the Eritrean capital Asmara, he conceded that expectations might have to be lowered on the open-ended talks U.N. Secretary-General Ban Ki-moon originally billed as "final."

Eliasson said invitations had only been sent out a week ago and he did not know which among more than a dozen rebel groups would come. "It's a moving target right now. ... I don't have the final list yet," he said.

HEAT ON REBELS

International experts estimate 200,000 people have died and 2.5 million have been driven from their homes in the Darfur conflict, which began with a revolt against the Sudanese government, but Khartoum puts the death toll at 9,000.

Trying to win over undecided groups, Eliasson said: "I want to very clearly make the point that the consultations that they are carrying on now ... will of course also continue in Sirte.

"The real negotiations will start only after we have the full preparations of the parties," he said.

Sudan's government has pledged to attend the talks and its U.N. ambassador welcomed the Council statement as turning the heat on rebels after previous statements faulting Khartoum.

"It's good that they criticize now and that they point their fingers towards the rebels," Abdalmahmood Abdalhaleem told journalists.

Eliasson said he now had "very little hope if any" that Sudan Liberation Movement founder Abdel Wahed Mohamed el-Nur, who has a wide following in Darfur refugee camps, would come. El-Nur has demanded a string of concessions from Khartoum.

Eliasson also noted that Khalil Ibrahim, leader of the main branch of the Justice and Equality Movement, had asked for a month's delay. But he said he had assurances that Ibrahim would send high-level representatives to Sirte.

The Security Council statement also called on U.N. member states to urgently provide aviation and ground transport units still needed for a 26,000-strong U.N.-AU peace-keeping force that will assume its duties next year in Darfur.

U.N. officials are worried that a lack of such specialized units, especially helicopters, could delay the deployment.

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