The UN has evacuated all "non-essential" staff from the Chadian capital to Cameroon.
France flew a combat unit of 126 extra troops into Chad to join the 1,100 permanently posted there.
Ban Ki-moon, the UN secretary-general, visiting Kenya on Friday, said he was deeply concerned by the resumed fighting in Chad and expressed his "condemnation of the use of military means to seize power".
He called on "all parties to abide by their commitments under the different peace accords signed by them and to urgently resort to dialogue to reach a peaceful and negotiated settlement of this latest crisis".
Peace pact collapse
Erdimi allied himself with other rebel leaders Mahamat Nouri and Adbelwahid Aboud Makaye in mid-December, after a peace pact with Idriss Deby, the Sudanese president, fell apart.
As a result, Chad now accuses Sudan of uniting and arming the rebel fighters.
|Clashes have delayed the deployment of |
Eufor peacekeepers to Chad [AFP]
On Monday, a rebel convoy of 300 pick-ups, each capable of carrying between 10 and 15 men, approached Ndjamena after leaving bases across the border in Sudan's western Darfur region, in the biggest such push since April 2006.
On Friday, the European mission, Eufor, announced a temporary delay in troop flights to Ndjamena, one with a dozen Austrian soldiers and two with around 50 Irish soldiers and equipment.
Philippe de Cussac, Eufor lieutenant-colonel, said: "The planes will not leave as long as the instability continues, but this does not mean it will last a long time."
At full strength, the force will consist of 3,700 men. They will protect about half a million civilians and humanitarian and relief work in Chad and Central African Republic.
General Jean-Philippe Ganascia, the French Eufor commander in Chad, said he was unconcerned by the rebels unless "they threaten or attack civilians, or the non-governmental organisations, or UN personnel".
Ban deplored the effect of the fighting on relief efforts in eastern Chad, which hosts about 240,000 refugees from Sudan's neighboring violence-torn region of Darfur, along with 179,000 displaced eastern Chadians and 43,000 Central Africans uprooted by strife and rebellion in the north of their country.
Ahmat Allami, Chad's foreign minister, spent several hours in talks with his African counterparts and European delegations on the sidelines of an African Union summit in Addis Ababa on Friday.