By Stephanie Hancock

N'DJAMENA, Nov 28 (Reuters) - Chadian rebels warned a European Union peacekeeping force bound for eastern Chad on Wednesday not to side with President Idriss Deby, saying they would fight it as a foreign occupation army if it did so.

The warning from the rebel Assembly of Forces for Change (RFC) followed the biggest battle in months in eastern Chad between Deby's forces and fighters from another major insurgent group, the Union of Forces for Democracy and Development (UFDD).

Both the UFDD and the government said their soldiers had killed hundreds of enemy combatants in clashes on Monday near the border with Sudan's Darfur region. The fighting shattered a month-old peace deal signed between Deby and his rebel foes.

The UFDD and RFC rebel groups, allies in a 2-year-old eastern insurgency against Deby's 17-year rule, abandoned a ceasefire at the weekend and accused the government of not honouring parts of the Libyan-brokered peace accord.

Against the backdrop of this renewed conflict in east Chad, up to 3,700 EU peacekeepers are due start deploying there early in the New Year on a mission to protect thousands of Sudanese and Chadian refugees and foreign aid workers who care for them.

"If this force is coming to secure the refugee camps, then we see no problem," RFC leader Timane Erdimi told Reuters.

"But if they're coming to help Idriss Deby's regime, we'll consider them a foreign occupation force and we'll fight them," he added, speaking by satellite phone.

Erdimi, a nephew and former close aide of the Chadian president who defected to join the eastern insurgency, said RFC forces were maintaining their positions in a mountainous area south of Guereda in eastern Chad.

"If the government troops attack us, we'll respond," he said, but added that if left alone his group would be willing to discuss trying to salvage the peace agreement.

The UFDD, led by another defector, former Defence Minister Mahamat Nouri, has also in the past warned the planned EU force not to try to come between its fighters and the government army.

The latest battle has once again stoked tensions between uneasy neighbours Chad and Sudan and highlighted the risks and sensitivity of the EU deployment, which will be the largest mission in Africa undertaken to date by the European bloc.

DOUBTS AND DELAYS

Chad on Tuesday summoned the Sudanese ambassador to deliver a protest, saying the UFDD rebels had attacked from Sudan. Both governments have in the past accused each other of supporting hostile armed groups, and last month's peace deal signed with Libya's backing was the latest accord to try to stop this.

The Chadian presidency accused Nouri's UFDD of seeking to delay or torpedo the planned EU deployment.

"Nouri and his mercenaries were very quick to reject the coming deployment of the 4,000-strong European force in eastern Chad, a force that will seriously compromise their warlike plans," said an editorial posted on the presidency Web site.

Apart from the difficulty of its mission, the EU deployment has already been delayed as its organisers struggle to raise a full complement of troops and equipment from member states.

The force's Irish commander, Lt-Gen. Pat Nash, said this month he could not deploy successfully unless given the planes, helicopters and medical units needed for the mission.

Some analysts have questioned whether the EU force will be able to avoid taking sides in eastern Chad, since almost half of its total strength is being provided by the French military, which has been assisting Deby under a bilateral defence accord.

But EU commanders say the French troops, some already stationed in Chad, will be "rehatted" as U.N. blue helmets, and they pledge to remain neutral in the east Chad conflict to concentrate on protecting civilians and aid workers. (Additional reporting and writing by Pascal Fletcher in Dakar; editing by Daniel Flynn and Mary Gabriel)

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