A French pilot
French forces have continued to launch air strikes against Islamist militants in Mali and sent troops to protect the capital, Bamako.
French Defence Minister Jean-Yves Le Drian said rebels heading towards the central town of Mopti were attacked. A French pilot was killed, he said.
French troops were deployed on Friday after Mali's army lost control of a strategically important town, Konna, reports the BBC.
Islamist groups overran huge areas of northern Mali last year.
One of the groups, Ansar Dine, moved further south into Konna on Thursday. But Mali's government said forces recaptured the town after the French air strikes.
Ansar Dine responded by threatening France with reprisals because of its intervention.
In a separate development, West African bloc ECOWAS has authorised the immediate deployment of its troops to Mali.
In a statement, ECOWAS Commission President Kadre Desire Ouedraogo said the decision was made "in light of the urgency of the situation".
A government official in Niger told Reuters news agency around 500 troops would be sent while AFP reported that Burkina Faso was sending a similar force.
The surge in fighting is the most serious since armed groups, some linked to al-Qaeda, took control of the whole of northern Mali last April.
Since then, two groups - Ansar Dine and the Movement for Unity and Jihad in West Africa (Mujao) - have sought to enforce an extreme interpretation of Islamic law in the area.
Regional and Western governments have expressed growing concern about the security threat from extremists and organised crime.
France's intervention in Mali began late on Friday afternoon, when helicopters were said to have opened fire on rebels heading south near Mopti.
The defence minister said on Saturday that hundreds of French troops were involved and many of them had been sent to the capital to provide security.
The minister said Paris had decided to act urgently to stop the Islamist offensive, which threatened to create "a terrorist state at the doorstep of France and Europe".
He also revealed that a French pilot was killed in Friday's fighting - during an air raid to support Mali's ground troops in the battle for Konna.
"During this intense combat, one of our pilots, Lieutenant Boiteux from the 4th helicopter regiment special forces, was fatally wounded," the minister said.
Hit by small arms fire, he had been taken to hospital in Mopti where he died of his wounds.
Speaking on Friday, French President Francois Hollande said the intervention complied with international law and had been agreed with Malian interim President Dioncounda Traore.
It would last "as long as necessary", Hollande said.
French officials gave few operational details.