By Claudia Parsons

UNITED NATIONS, Oct 21 (Reuters) - The United Nations is intensively lobbying countries to provide helicopters for a U.N.-African Union peacekeeping force in Darfur, one of many obstacles to starting the mission effectively on Jan. 1.

Officials and diplomats say no country has made a credible offer to provide the 24 transport and attack helicopters needed for the 26,000-strong force, whose mission is already clouded by lack of full commitment by the Sudanese government.

The force is due to replace a hard-pressed AU operation, known as AMIS, which lacks experience, equipment and cash and has been unable to stop the 4 1/2-year long conflict.

"No national army would deploy anywhere without any air power," one U.N. diplomat said. "The AMIS force there now has no helicopters, which is ... why it can't defend itself."

At least 10 AU troops were killed last month in an assault on their base in Darfur -- Sudan's arid western region the size of France where international experts estimate 200,000 have died and 2.5 million have been driven from their homes.

A Security Council resolution in July called on member states to finalize contributions to UNAMID, the United Nations-African Union Mission in Darfur, by the end of August.

With no helicopters secured six weeks after that, Britain wants the U.N. Security Council to issue a statement this week urging member states "to urgently make available the aviation and ground transport units still required."

"The situation we really, really have to avoid is one where the force commander will have not much more at his disposal than he has now," a U.N. official said.

Sources say among countries the United Nations is lobbying for air assets or to put pressure on Khartoum to cooperate with the force are Egypt, Pakistan, Ukraine, South Africa, South Korea and China, which is Sudan's most powerful close ally.

SUDAN'S RELUCTANCE

The Save Darfur Coalition, an umbrella group of 180 religious and human rights groups, wrote a letter to China's president on Thursday asking Beijing to provide helicopters.

The new mission will cost more than $2 billion a year. Itstask is daunting, with limited water supplies, sand storms and the nearest seaport -- Port Sudan -- more than 1,200 miles (2,000 km) from Darfur.

An aversion by Sudan to Western participation in the force means the United States, Britain and other European Union and NATO members are unlikely to offer helicopters, although the U.S. military will continue helping with logistics such as air-lifting troops.

Richard Grenell, spokesman for the U.S. mission to the United Nations, said Washington had paid to build and maintain 34 Darfur base camps for the AU force and contributed nearly $4 billion in assistance since 2005.

The Security Council resolution specified the force would be "predominantly African." Exactly what that means has proved controversial.

Sudan has yet to officially approve a force composition plan proposed by the United Nations, which would be 80 percent African in total, with 95 percent of the infantry African.

Norway and Sweden have offered around 400 army engineers, the Netherlands has offered a medical unit of 75 people and several Asian countries, including U.N. peacekeeping stalwarts Bangladesh and Nepal, have offered other contributions.

Theoretically it is up to the United Nations and the African Union to decide who will contribute. But diplomats admit privately that Khartoum has a de facto veto because the force will not be able to operate without Sudan's cooperation.

The U.N. diplomat said there were already concerns about Khartoum not providing enough land and about restrictions on night flights which were hampering logistical preparations.

Sudan's U.N. ambassador did not respond to repeated requests to discuss the peacekeeping force.

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