Thousands of people are fleeing the Chad capital, Ndjamena, after two days of fierce fighting between government and rebel forces in the city.
French military vehicles are already in Chad
The government says it has pushed the rebels out of the city but they say they withdrew to give civilians the chance to evacuate.
Aid workers report that fighting is continuing outside the city, while dead bodies litter the streets.
The UN Security Council has urged member states to help the government.
The BBC's Laura Trevelyan at the UN in New York says this non-binding statement gives the go-ahead to France and other countries to help President Idriss Deby's forces against the rebels.
June 2005 - Constitutional changes approved allowing president to stand for third term
April 2006 - Hundreds killed as rebels fight government troops on outskirts of Ndjamena
May 2006 - President Deby wins election boycotted by opposition
January 2008 - EU approves peacekeeping force to protect Darfur refugees from violence in Chad
Chad's former colonial power France has a military base in Chad and has previously helped the government with logistics and intelligence.
Thousands of people have been streaming across the Ngueli bridge, which separates Chad from Cameroon.
Local officials have told the UN refugee agency that thousands were also crossing at the border town of Kousseri.
"We're expecting a lot more people coming," said UNHCR spokesman Ron Redmond.
He also said he was extremely concerned for the 240,000 Darfur refugees in Chad.
A plane chartered by the French government carrying 363 foreigners evacuated from Chad arrived in Paris on Monday morning.
Others have gone to Gabon.
France's UN ambassador Jean-Maurice Ripert said it was too early to say whether France would provide military to the government.
"I guess there will be some request from the government of Chad addressed to some member states or to all member states of the UN and we'll see," he said, the Associated Press news agency reported.
"What is important is that the Security Council allows the member states to do so and to answer the request for help and assistance of Chad."
The rebels have previously threatened to attack a French-dominated European Union peacekeeping force, because of France's support to the government.
The deployment of that force has been delayed because of the latest fighting.
General Mahamat Ali Abdallah, who is commanding the government forces, said the rebels had been "completely routed".
But rebel spokesman Abderaman Koulamallah rejected this.
"We have pulled out of the city and we are waiting for the civilian population to be evacuated," he told AFP news agency.
"We certainly will go back on the offensive... We're asking the civilian population of Ndjamena to leave immediately because their safety cannot be assured."
'Window on genocide'
The army has also said it had thwarted a second rebel attack on the town of Adre, near the border with Sudan over the weekend.
This is where the refugees from Darfur are based, living in camps and where the EU force is due to deploy.
The EU force is intended to protect refugees from the Darfur region of neighbouring Sudan, as well as aid workers.
Chad accuses the Sudanese government of backing the rebel offensive in Chad in order to stop the EU force from being sent to the region.
THE REBEL COALITION
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
"Sudan does not want this force because it would open a window on the genocide in Darfur," said Foreign Minister Amad Allam-Mi.
Sudan denies this, as well as accusations that it has supported Arab militias accused of ethnic cleansing and genocide in Darfur.
Chadian rebels seized control of large parts of the capital on Saturday, approaching the palace where President Deby was holding out.
Mr Deby seized power in a coup in 1990, but has won three elections since then, although their legitimacy has been challenged.
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