French troops based in Chad also took up positions around government office buildings late on Wednesday in anticipation of a rebel attack. France, which supports longtime President Idriss Deby's government, responded by sending 150 troops to add to its contingent of about 1,200 already in Chad to protect about 1,500 French citizens there.

The troops are being dispatched from the central African country of Gabon, the French Defense Ministry said in a statement in Paris.

Mercenaries

The government describes rebel groups in Chad as mercenaries and militia working on behalf of the government of neighbouring Sudan, an accusation Sudan denies.

Scores of defectors from the Chadian army have joined rebel groups in their effort to overthrow Deby, who seized power in a 1990 coup and has seen his authority undermined by violence in neighboring Sudan and an apparent struggle for control of newly discovered oil reserves.

The troubles in Chad have revived fears that the Darfur conflict in neighboring Sudan has the potential to undermine the region where Sudan, Chad and the Central African Republic meet.

Since October, the rebels have been skirmishing with government forces along the border, which is more than 1,000km east of N'djamena.