French troops and aircraft are stationed in Chad under a bilateral defence accord. The EU force, around half of which will be French, is preparing to deploy near the eastern border with Sudan in coming weeks to protect refugees and aid workers.

The Chadian rebel group Union of Forces for Democracy and Development (UFDD) said in a statement that it now "considers itself to be in a state of war against the French army, or against any other foreign forces in the national territory".

UFDD fighters have been battling government forces loyal to President Idriss Deby in eastern Chad since the weekend, in fierce clashes that have shattered a month-old peace accord between Deby's government and his main rebel foes.

The rebels said French military aircraft had flown over their positions on reconnaissance missions for the government during heavy fighting on Thursday between the towns of Guereda and Adre, along the border with Sudan's Darfur region.

"Providing diplomatic, strategic and logistical support to the tyrant Idriss Deby is an act of hostility and will be treated as such," the UFDD statement sent to Reuters said.

Rebels have been fighting Deby in the east for two years. They have in the past criticised France's support for him, but have stopped short of direct hostilities with French forces.

Chad is a former French colony, and France is providing around half of the 3,700 EU peacekeepers who are due to start arriving early next year on a U.N. mission to protect camps housing more than 400,000 Chadian and Sudanese refugees.

The EU force for Chad, which will also send soldiers to the northeast of the Central African Republic, is intended to try to help contain a widening conflict in Sudan's Darfur region, which has pushed armed raiders and refugees across the border.

It will complement a bigger United Nations/African Union peacekeeping force planned for Darfur, where political and ethnic conflict triggered by a 2003 rebellion has killed at least 200,000 people, experts say.