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 army units had attacked a column of rebels heading towards the central town of Mopti.
He also revealed that a French pilot had been killed in fighting on Friday.
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The French troops were deployed on Friday after Mali's army lost control of a strategically important town.
Mali's government said its forces had recaptured the town, Konna, after the air strikes, but it was not clear if all Islamist fighters had left the area.
Armed groups, some linked to al-Qaeda, took control of the whole of northern Mali in April.
They 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.
Mr Le Drian said on Saturday that hundreds of French troops were involved in the military operation in Mali.
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... was fatally wounded," the minister said.
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", Mr Hollande said.
French officials gave few operational details.
Residents in Mopti, just south of Konna, told the BBC they had seen French troops helping Malian forces prepare for a counter-offensive against the Islamists.
Mr Traore declared a state of emergency across Mali, which he said would remain in place for an initial period of 10 days.
He used a televised address to call on Malians to unite and "free every inch" of the country.
The west African bloc Ecowas said it was authorising the immediate deployment of troops to Mali "to help the Malian army defend its territorial integrity".
The UN had previously approved plans to send some 3,000 African troops to Mali to recapture the north if no political solution could be found, but that intervention was not expected to happen until September.
French Foreign Minister Laurent Fabius said the aim of the operation was to stop Islamist militants advancing any further.
It was not clear how far the French would go in helping Mali's government retake territory in the north.
At least seven French hostages are currently being held in the region, and Mr Fabius said France would "do everything" to save them.
A spokesman for al-Qaeda in the Islamic Maghreb (AQIM) said he considered the French operation a "Crusader intervention", and told France it would be "would be digging the tombs of [its] sons" if the operation continued, according to the Mauritania-based Sahara Media website.
France ruled Mali as a colony until 1960. - BBC