(Adds background on EU force, French defence minister)

By Betel Miarom

N'DJAMENA, Oct 3 (Reuters) - Four Chadian rebel groups initialled a peace agreement with the government on Wednesday at talks in Libya, a Chadian official said, but the leader of the main faction said there were many points left to resolve.

"The contents are secret. An agreement should be officially signed very soon in a ceremony that will bring together heads of state in Tripoli," a senior Chadian government official, who asked not to be named, told Reuters in Chad's capital N'Djamena.

The talks have dragged on for weeks but have taken on greater urgency as the European Union assembles a peace force to deploy in the conflict zone in eastern Chad to help stem violence spreading from neighbouring Sudan's Darfur region.

In arid eastern Chad, where refugees and violence have spilled over the border from the four-year-old war in Darfur, local Chadian rebel groups have waged a cat-and-mouse rebellion against Chadian President Idriss Deby.

The main rebel movement's leader involved in the deal, Mahamat Nouri, was cautious about the extent of the agreement, saying "many, many, many" points remained to be thrashed out.

These included creating conditions for rebels to disarm in safety and their participation in state affairs, he said.

"We're moving very slowly," he told Radio France International.

Besides Nouri's Union of Forces for Democracy and Development (UFDD), three other rebel groups were involved in Wednesday's preliminary agreement in Tripoli.

Chad's Minister of State for Infrastructure Adoum Younousmi signed for the government side, the Chadian official said.

Another small rebel faction signed a peace deal with Deby's government in N'Djamena on Monday, which had appeared to indicate progress at the talks sponsored by Libyan leader Muammar Gaddafi. A new round of Darfur peace talks are due to start in Tripoli on Oct. 27.

Those talks were called at the request of U.N. Secretary-General Ban Ki-moon as the United Nations and African Union assemble a 26,000-strong hybrid force to replace an overstretched, under-resourced African Union force in Darfur.

To complement the Darfur force, the European Union is assembling a separate force of between 2,500 and 4,000 EU troops to protect civilians and aid operations in Chad and northeastern Central African Republic, which have both been affected by violence spreading from Darfur.

French Defence Minister Herve Morin told Reuters television in Paris on Wednesday that he was sure that EU countries will contribute at least 3,000 troops for a peacekeeping force in eastern Chad and Central African Republic.

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