Rebels in Chad are advancing from the east of the country towards the capital, N'Djamena, after seizing a strategic central town, officials say.
Rebels said a coalition of groups had launched the attack
Armed forces have gone to intercept a column of 300 rebel vehicles, advancing along the main road to the capital.
Security has been stepped up in N'Djamena, where most children have been sent home from school.
The rebels want to drive the president from power. Two years ago they reached the capital before being repelled.
A spokesman for several rebel groups who have joined forces said Oum Hadjer, a strategically important town in the central region of Batha, 400km (248 miles) from N'Djamena, had been captured on Wednesday.
A series of armed attacks on the UN refugee agency and other aid organisations has led to the evacuation of staff from the UNHCR office in Guereda in the east.
A European Union peace force is preparing to deploy to eastern Chad in the next few weeks.
The 3,700-strong contingent will aim to protect refugees from Sudan's Darfur region and people displaced by internal fighting.
The Chadian military said a convoy of rebel forces had been sighted in the area around Oum Hadjer, but that there was conflicting information about its movements.
A spokesman for the rebel group, the Union of Forces for Democracy (UFDD), Mahmat Hassan Boulmaye, told the BBC they wanted to overthrow President Idriss Deby's government.
"Our troops are at 200km from N'Djamena. So far there hasn't been any clash with government forces, but this may happen soon, as the army has been sent to meet our troops. So it may well happen in the near future."
Meanwhile, army units have stepped up patrols on the streets of the capital, Reuters news agency says.
France's embassy has closed the French school in the city, French radio reports.
This comes after Chadian aircraft bombed rebel positions in the east near the border with Sudan on Tuesday.
The peacekeeping 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.
France already has 1,100 soldiers in Chad under a 1986 agreement to guarantee "territorial integrity", but rebel groups believe the French helped thwart the attempted N'Djamena attack in 2006.
Rebels maintain the Eufor Chad/CAR will not be neutral.
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.