"Biltine has just fallen," Ali Gadaye, spokesman for the anti-Deby National Alliance, told Reuters by telephone.
There was no immediate reaction from the government or independent confirmation of the fall of Biltine to the fighters, who have said their aim is to advance towards the capital N'Djamena, some 700 km (450 miles) to the west.
Biltine is the third eastern town which the fighters have attacked in the last few days in what they say is a major military advance to try to overthrow Deby, whose rule they denounce as corrupt and dictatorial.
Chad's government, while acknowledging rebel columns are on the move in the east, have played down the extent of the insurgent offensive, calling it "rebel propaganda".
Biltine lies just over 90 km (55 miles) north of Abeche, the main hub of international humanitarian operations in eastern Chad, which borders with Sudan's Darfur region.
Troops from both France and from a European Union military force (EUFOR) are based at Abeche.
On Saturday, a column briefly occupied Goz-Beida, more than 200 km (125 miles) south of Abeche, witnesses said.
On Sunday, Chad's government said another rebel column "passed through" lightly-protected Am-Dam 140 km (85 miles) southwest of Abeche. The fighters said they briefly occupied Am-Dam, meeting little resistance.
Güncelleme Tarihi: 16 Haziran 2008, 17:23