Several columns of rebels seeking to overthrow the Chadian government attacked the eastern relief and military hub Abeche before dawn on Saturday, Chadian authorities and the insurgents said.

Authorities said that the army had deployed around Abeche, a dusty low-rise city that holds considerable strategic importance in the sparsely populated desert region. The rebels claimed to control the town. This could not immediately be independently confirmed as communications to the city were shut off on Friday.

“The Army Chief of Staff calls on the population to remain calm," said a statement released on Saturday attributed to the Chadian Army Chief of Staff Adoum Guelemine Gabgalia.

“What Abeche is living now is a repeat of the battle that unfolded last April in N’djamena,” the statement also said, referring to an attack by hundreds of rebels on Chad's capital N'djamena that saw more than 200 people killed in a morning of fierce fighting on the city’s outskirts.

The April attack was repelled after French fighter jets bombed advancing rebel columns. France has a large military base on Abeche’s outskirts, and regularly provides aerial surveillance and support to Chad’s army loyalists, but has never directly engaged the rebels.

The government of President Idriss Deby has experienced waves of defections from the army since Deby held and won a widely criticised referendum last year that allowed him to change the constitution to run for a third term. Chad’s political opposition boycotted the subsequent elections held in May this year and Deby was declared victor.

Chad’s army loyalists have been involved in over a dozen skirmishes and major battles with the rebels, which operate from camps in the lawless Darfur region of neighbouring Sudan and northern Central African Republic to Chad’s south. Sudan denies this.

After a lull in fighting during Chad’s rainy season, in September the Chadian army engaged the rebels in the Aram Kolle mountains north of Abeche. Last month the rebels briefly held the towns Goz Beida and Am-Timan some 600 km south of Abeche. Chad’s army reclaimed both towns and the rebels slipped away into the open desert after a battle that packed the region’s hospitals with scores of civilian, military and rebel casualties.

It is not known how large the rebel groups are and they have released little information about their goals, beyond removing Deby from office. The movement has dozens of different wings and constantly shifting coalitions.

Chad has experienced decades of devastating civil war since gaining independence from France in the 1960s. Diplomats in N’djamena have said that they fear Chad’s numerous clans and ethnic groups could return to hostilities if Deby, a hard nosed former military chief who himself seized government in a coup in 1990, is no longer in the top spot.

While Chad’s army has focused on fighting the anti-government rebels, militia groups in Sudan have taken advantage of the open desert border to launch armed attacks on Chadian border villages.

At least 75,000 people have fled their villages in the past year - 12,000 of them this month alone, according to the United Nations refugee agency (UNHCR), and hundreds have been killed in well-documented massacres.

Chad has accused Sudan of seeking to destabilise Chad by exporting conflict from Darfur and by supporting the rebel groups opposed to the government.

The Sudanese government in Khartoum denies both accusations, and has accused Deby of backing rebel groups opposed to it.