The Rally for Democracy and Liberty (RDL), opposed to Chad's president, Idriss Deby, said in a statement on Wednesday that its forces had made a "tactical withdrawal" from the town of Adre on Chad's eastern border with Sudan after intense fighting on Sunday.
Chad's government says its army repulsed two attacks by the rebels on Adre, pursued them over the border into Sudan and destroyed their bases there. It accused Sudan of supporting the attackers and said around 300 of them were killed.
Speaking in el-Geneina, in Sudan's western Darfur region, Abdullahi Abdel Karim, a RDL official, rejected the Chadian government's version of the fighting as contradictory and exaggerated.
"RDL fighters are at the gates of Adre and occupy all the main approaches leading to the town. The next assault will be decisive," he said in what was the rebel group's first known public comment on the fighting.
Abdel Karim said more than 70 Chadian government troops were killed and about 50 wounded in the battle over Adre.
The fighting over Adre has raised tensions in neighbouring Darfur where Sudanese rebels have the country's central government for almost three years.
Chad has accused the Sudanese president, Omar Hassan al-Bashir, and his government of sheltering and equipping armed opponents of Deby "as though they were part of the Sudanese army". It said it will pursue them inside Sudan if necessary.
Fighting in eastern Chad and
Darfur is a growing concern
Scores of Chadian soldiers deserted their barracks in late September before regrouping near the border and joining up with other rebels to attack the government.
Abdel Karim repeated the fighters' demand that Deby should step down.
"Any solution other than the departure of Chadian President Idriss Deby is impossible," he said.
The international community is showing growing concern about the fighting in eastern Chad and in Darfur, a region the size of France where tens of thousands have been killed and more than two million people have been forced to flee their homes.
Kofi Annan, the UN secretary-general, has said that the region is descending into anarchy.The US said anti-government forces in Chad could launch fresh attacks and informed American citizens about the dangers of travelling there.