N'Djamena - Chad's army has driven the rebel Union of Forces for Democracy and Development (UFDD) back to its base in Sudan, where a rebel leader said on Thursday the movement was awaiting the "decisive battle".

"Government forces on Tuesday attacked our forces in two columns," UFDD vice president Acheikh Ibn Oumar has said, adding that one army column "pushed right up to our headquarters about a dozen kilometres (seven miles) inside Sudan."

With the other offensive at Am Zoer in Chad, "these attacks have forced us to regroup our forces to defend our headquarters, which we have done," Ibn Oumar said, reached by satellite telephone.

"This morning, the forces of President (Idriss) Deby have pulled back, but he is regrouping his troops on the border," the rebel leader said. "He is determined to eliminate us. The next battle will be decisive."

The Chadian government claimed on Wednesday that the UFDD, led by former defence minister General Mahamat Nouri, and a separate rebel coalition, the Rally of Democratic Forces (RAFD) had been "totally annihilated".

The rebels denied this and also gave different casualty figures, but it was clear that the government army had gone on to the offensive, apparently led by President Idriss Deby Itno himself, who was said by military sources to have set up base in the border town of Adre.

In N'Djamena, Communications Minister Hourmadji Moussa Doumgor said that 700 rebels had been killed and "a very large number" wounded in fierce fighting since Saturday, when there was a battle round the eastern Chad town of Biltine.

Ibn Oumar said Thursday that in fighting on Tuesday, about 100 government soldiers had been killed, while the UFDD lost 15 men and "at least 35 were wounded".

"The fighting was very heavy, but we didn't back off," he said.

"We recovered the personal vehicle of the defence minister, along with some documents belonging to him, and those of three other army generals. In all, we destroyed 30 enemy vehicles and captured about 10," he added.

The rebel leader said Deby was so determined to crush the insurgency that "he is recruiting gendarmes, customs officers, even forest rangers."

The UFDD and the RAFD late in October launched attacks in eastern Chad and briefly took towns including the region's main one, Abeche, over several weeks, each stating that their strategy was not to keep towns but to wage a war of attrition against Deby's forces.

The RAFD, led by twin brothers and Deby nephews Tom and Timane Erdimi, and the UFDD both came under attack on Tuesday at Hadjer Marfain on the frontier with Sudan's strife-wracked Darfur region.

On Wednesday, RAFD spokesperson Yaya Dillo Djerou said that N'Djamena's claims were "totally false", but acknowledged that one of the rebel coalition leaders had been killed.

"Our chief of staff Seby Aguid died on Tuesday, that's true, and they are trying to spread disinformation focussed on that," Djerou said.

Deby and his counterpart in the Central African Republic, President Francois Bozize, have jointly accused Sudan of backing rebel forces in their countries, while relief agencies fear an overspill of the deadly ethnic conflict wracking Darfur.

Khartoum in turn accuses Chad and CAR of supporting rebels in Darfur.