UNITED NATIONS, Dec 6 (Reuters) - U.N. Secretary-General Ban Ki-moon said on Thursday that a plan to send 26,000 peacekeepers to Sudan's Darfur region was at risk because U.N. members were failing to contribute essential helicopters.

In a statement to journalists, Ban called on member states to "walk their talk" about the need for the force to deploy by supplying the transport and attack helicopters U.N. planners say they must have.

He said the force was needed to entice Darfur rebel groups into peace talks with Khartoum. "But for this we need on-the-ground capability -- specifically helicopters. We're not getting them. Because of that the entire mission is at risk."

On Jan. 1, the joint U.N.-African Union force is due to take over from an existing AU force of just 7,000, which has been unable to stop a conflict that has raged for 4-1/2 years.

Diplomats say countries that could contribute the 18 transport and six attack helicopters the United Nations wants have been reluctant to do so either because they have none to spare or because they are unhappy about the new force's command and control arrangements.

The U.N. chief said he had personally contacted "every possible contributor", including on trips to South America and Europe and at last week's Middle East conference in Annapolis, Maryland -- but to no avail.

Ban said he had sent a letter to the Security Council on Thursday calling on its 15 members "to live up to their responsibility".

"We are at the critical moment for Darfur. Time is running out. Troops begin deploying in January. Member states have spoken clearly about what must be done. It is time for them to walk their talk," he said.

(Editing by Philip Barbara)

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