France has warned rebels in Chad that it could intervene if they continue to threaten the capital, N'Djamena.
Foreign Minister Bernard Kouchner said he "hoped" the French troops in the city would not have to intervene.
President Nicolas Sarkozy said France would "do its duty" if it must. The rebels have reportedly accused the French of bombing them already.
The rebels seized much of the city over the weekend but the government is now in control. Thousands have fled.
Mr Kouchner was speaking after the United Nations Security Council called on all member-states to back the Chadian government.
" I don't know whether to cry or kill myself" Chad refugee's escape
Former colonial power France has 1,400 troops in Chad and has helped to evacuate foreigners from N'Djamena.
Meanwhile, the AFP news agency reports the rebels are willing to accept a ceasefire.
'Think again'
"We really hope we won't have to intervene further," Mr Kouchner said.
But "when it came to defending the [N'Djamena] airport, our forces did so very sharply, very effectively," he said.
Q&A: Rebellion in Chad
President Sarkozy reiterated France's determination not to allow rebels to overthrow the government.
"If France has to do its duty, it will. No-one should doubt that," he told reporters in western France.
Rebel leader Mahamat Nouri told French radio that their positions had been bombed throughout Monday.
He said the rebel forces would launch a new battle for the city unless the French intervene.
"Because of yesterday's UN resolution giving the French a mandate to intervene, we will have to think again," he told Europe 1 radio, AFP reports.
Some 20,000 people have estimated to have fled from N'Djamena to neighbouring Cameroon.
Aid officials say they are struggling to help those who are crossing into the town of Kousseri, just across the Chari River from the Chadian capital.
The man in charge of United Nations relief operations in the town, Gilbert Lubaki, told the BBC they were aiming to reopen a camp 30km away, which had been used in previous emergencies, and organise food distribution and shelter there.
At the moment, he said, many of the refugees were scattered in local schools in Cameroon and surviving on what they had brought with them.
The UN refugee agency says it is preparing to send two airlift flights this week from its regional aid stockpiles in Dubai to Cameroon.
African Union mediators are due in the city to try to end the fighting.
The fighting has led the European Union to delay sending its peacekeeping force to Chad. More than half of this 3,700 strong force will be French.
The force was meant to protect refugees from Darfur and aid workers.
The latest rebel offensive began on the day the first troops were meant to arrive.
Chad has accused Sudan of backing the rebels to stop the peacekeepers getting too close to Darfur.
Sudan denies the charges.
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