By Stephanie Hancock

N'DJAMENA, Dec 7 (Reuters) - Rag-tag insurgents in eastern Chad are taking advantage of an army offensive against larger rebel groups to attack weakened garrisons near the Sudan border, compounding unrest that is disrupting aid operations.

Military officials in the former French colony said on Friday two previously little-known rebel groups calling themselves the FSR and FPRN had attacked an outpost at Tissi near the borders with Central African Republic and Sudan.

"They attacked the garrison at Tissi on Thursday morning. They knew that government forces had left to go to (the nearby towns of) Am Timan and Haraz Mangueigne so they took advantage," one military official said, asking not to be named.

He said two military police officers protecting the garrison were killed but another managed to raise the alarm. The army then sent two heavily armed convoys to the area and a two-hour firefight ensued.

A second military source said at least 60 government soldiers were wounded.

Chad's army has been battling at least three main rebel groups further north in the heaviest fighting in months along the border with Sudan's Darfur region after an Oct. 25 peace deal with President Idriss Deby collapsed.

Both sides claim to have killed hundreds of their enemies.

Government soldiers involved in some of the main fighting appeared exhausted when journalists visited a battle zone northeast of the town of Biltine this week.

Independent casualty figures are difficult to obtain, but a senior humanitarian worker in the capital N'Djamena told Reuters that hundreds of fighters had been killed and wounded since fighting resumed late last month.

The renewed conflict comes just weeks before a European Union peacekeeping force of up to 3,700 soldiers is due to deploy here on a United Nations mission to protect several hundred thousand refugees and the aid workers caring for them.

One of the main rebel factions, the Union of Forces for Democracy and Development (UFDD), has declared a "state of war" against French and foreign military forces in an apparent warning to the EU force.

Military sources said the FSR faction involved in the assault on Tissi has also publicly stated its opposition to the arrival of European peacekeepers.

Relief agencies trying to care for hundreds of thousands of displaced Sudanese and Chadian civilians say the unrest is hampering their ability to provide aid.

"Access to some refugee camps has been impossible for days and aid workers are unable to operate because of the insecurity," the U.N. World Food Programme, which is supplying monthly rations to more than 230,000 refugees from Darfur and almost 150,000 displaced Chadians, said in a statement. (Writing by Nick Tattersall; editing by Alistair Thomson)

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