French military aircraft have been patrolling the Chad-Sudan border to ensure there has been no interference in the fighting around N'Djamena.
France's defence minister said it wanted to monitor "any self-styled foreign intervention" in the fighting between Chad's government and rebels.
The government has accused Sudan of giving the rebel groups rear bases in Darfur, a charge which Khartoum denies.
Thousands have fled N'Djamena since the rebels launched an assault on Saturday.
Up to 20,000 people have crossed the river border with Cameroon in the past four days and arrived in the town of Kousseri, placing heavy strain on essential supplies and accommodation, the UN refugee agency has said.
More than 3,000 other refugees have fled to Nigeria.
Earlier, the leader of the main UFDD rebel group said it was prepared to have a ceasefire in return for the promise of negotiations with the government, but the government dismissed the offer, saying it had already beaten the rebels.
A mediation mission from Libya and Congo-Brazzaville, appointed by the African Union with a brief to meet both sides, is due to arrive soon in the Chadian capital.
Speaking about the role of his country's 1,400 troops based in Chad, French Defence Minister Herve Morin stressed that the UN declaration on Monday calling on all countries to support the government had not changed the terms of engagement.
"What it does do is give international community support to the actions of France," he told Radio France Internationale. "It is also support for [President] Idriss Deby."
"It is international community support for the integrity of Chad and support for the actions of France, actions that we've been carrying out for several days."
Mr Morin said that France did have a military agreement with Chad which provides for logistical, medical and training support, but "in no way is it a defence agreement... that would oblige France to intervene to protect the sovereignty of the country involved".
The French military could intervene if it was authorised to do so by a UN resolution, he added.
However, Mr Morin admitted that French fighter jets and reconnaissance planes had been flying over the border with Sudan over the past 36 hours in line with a request from President Nicolas Sarkozy to ensure there are no foreign incursions.
"It enables us to monitor and verify any self-styled foreign interventions and to date we've seen nothing," he said.
"What is certain is that these rebel forces were over by the Sudanese border," he added. "What we might well find out in the days ahead is just what the involvement of the Sudanese actually is."
The violence in the western Sudanese region of Darfur and the cross-border fighting between Chad and Sudan has in recent years sent at least 400,000 people fleeing to refugee camps in eastern Chad.
A French-dominated European Union peacekeeping force had been due to start deploying to eastern Chad last week to give the refugees and aid workers there a measure of protection, but the latest rebel offensive began at the same time.
BBC world affairs correspondent Mark Doyle says one theory is that Sudan encouraged the rebels to attack in order to stop the EU opening a window on Khartoum's activities in Darfur, where it has been accused of genocide.
Mr Morin's comments came as the leader of the largest rebel group, the United Force for Democracy and Development, told the BBC that it was prepared to agree to a ceasefire in return for the promise of talks with the government.
Mahamat Nouri said the ceasefire offer had been made by the Libyan leader, Muammar Gaddafi, and the government of Burkina Faso.
But the Chadian Prime Minister, Nouradine Delwa Kassire Coumakoye, was dismissive of the offer.
"Why a ceasefire?" he told French TV station France 24.
"They don't exist any more. With whom would we sign a ceasefire?... We've got them under control."
The lull in the fighting around N'Djamena following the tripartite rebel alliance's recent withdrawal has prompted tens of thousands of Chadians to flee the country.
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
On Tuesday, "frightened people were still crossing in a continuous flow" from Chad into neighbouring Cameroon, said the UN's refugee agency in a news release.
Thousands have deluged Kousseri in Cameroon, the UNHCR said. While some have found refuge with relatives, in schools or hotels, it said, between 6,000 and 7,000 were staying out in the open at a transit centre near the bridge.
The UNHCR said it planned to move these people to an old campsite some 30km away which could hold up to 100,000 people and was equipped with wells.