Fears for refugees
Annette Rehrl, from the UN refugee agency in Abeche, told Al Jazeera that the heavy fighting ended after several hours with each side claiming to have killed hundreds of soldiers.
She confirmed the clashes were close to refugee camps and added: "We are of evacuating people [civilians] off the ground and we hope that we will not be targets if clashes escalate."
The clashes took place near a major camp for Darfurian refugees, aid groups said, in a zone where European Union peacekeepers are scheduled to be deployed.
Mahamat Nouri, UFDD leader, said the fighting erupted near the main eastern city of Abeche and the frontier with Sudan.
Humanitarian workers at the Farchana refugee camp said they had heard heavy artillery being fired.
The unrest came after this weekend's collapse of a month-old peace accord between the government, the UFDD and the Rally of Forces for Change (RFC), another armed group.
Nouri has accused Idriss Deby Itno, the Chadian president, of ordering the attack on his fighters, saying: "Now that the fire has started, there is no more ceasefire."
His remarks appeared to contradict a joint statement from the groups released on Monday in Khartoum in which they said they wanted to "save" the October 25 peace deal which expired midnight on Saturday.
In the statement, the UFDD, the RFC and the UFDD-F (UFDD-Fundamental) claimed "their readiness to renew with all the arrangements that Sudan and Libya, the mediators, deem necessary to save the accord" and save lives.
The rebels said they would hold the Chadian government responsible for whatever followed due to its "irresponsible attitude."
The government accused the UFDD and the RFC of breaking the preliminary peace accord, signed in Syrte, Libya, on October 25, by crossing the Sudan border to attack the gendarmes.
Nouri and Timam Erdimi, the RFC chief, in turn accused Deby's government of failing to keep promises in the peace accord.