Rebels and Chad government troops have clashed, as insurgent forces advance towards the capital, N'Djamena, with both sides claiming victory.
Rebels said a coalition of groups had launched the attack
"We expect to be able to eat our lunch in N'Djamena," rebel spokesman Abderaman Koulamallah told the BBC.
The UN is pulling non-essential expatriate staff out of the city and France is flying in 150 extra troops to protect French nationals there.
A European Union force has delayed sending troops because of the clashes.
"Ongoing instability around N'Djamena has delayed the flights," Dan Harvey, a spokesman for the operation commander, told the BBC News website.
Mr Harvey said that the planes were due to land on Thursday night and Friday morning with more than 100 troops from Austria and Ireland.
The 3,700-strong contingent is supposed to protect refugees from Sudan's Darfur region and people displaced by internal fighting.
The government also claimed victory.
"The column of mercenaries in the pay of Sudan... has been completely put to flight... the battle is over, it's finished, we're in pursuit," Chadian Territorial Administration Minister Ahmat Mahamat Bachir told Radio France International.
THE REBEL COALTION
Unified Military Command includes:
Union of Forces for Democracy (UFDD) led by Mahamat Nouri
Rally of Forces for Change (RFC) led by Timane Erdimi
UFDD-Fundamental led by Abdelwahid Aboud Mackaye
The AFP news agency reports that access to the N'Djamena airport has been cut, quoting an Air France spokesperson, who said a flight from Paris had not been able to take off.
Mr Koulamallah said the rebel coalition - made up of a column of about 300 vehicles - had fought off government troops on Friday morning.
"The government's forces have just attacked our positions. We pushed them back, and at the moment we are progressing towards the city of Massaguet," he said.
A Chadian journalist has told the BBC that several sources report fighting between the town of Massaguet and the capital 80km (50 miles) away.
But Chad's ambassador to the United States, Mahamoud Bechir denied the government was on the verge of collapse.
"At the end of the day, I think they have committed a suicidal attempt, they have made a big mistake," the diplomat told the BBC's Network Africa programme.
"We're not at all threatened and the government is ready to give a lesson to them."
France says it is flying in extra troops to boost the 2,000 troops already in the city under a 1986 agreement to guarantee "territorial integrity".
France says the troops are to protect its nationals in its former colony.
Rebel groups believe the French helped thwart them in 2006, when they reached N'Djamena before being repulsed.
France is believed to have helped thwart the 2006 rebel attack
The EU force - known as Eufor Chad/CAR - to be deployed in Chad and the Central African Republic - was given final approval on Monday and will mainly contain French troops.
Late last year, the UFDD declared a "state of war" against French and other foreign forces because it said they were "bringing diplomatic, strategic and logistical aid" to the president.
Mr Bechir accused Sudan of backing the rebels, saying Khartoum was "nervous" about the deployment of peacekeepers in Chad and more UN and African Union troops in Darfur.
"The are competing with time to stop that," the ambassador said.
Sudan has denied backing the Chad rebels and in turn accuses President Deby's government of backing Darfur rebel groups.