French fighter jets bombed northern Mali as the French government sent more troops to the capital Bamako, awaiting a West African force to dislodge Al-Qaeda-linked insurgents from the country's north.
The attack on Gao, the largest city in the desert region, is said to be controlled by the Islamist alliance.
The air strikes marked a decisive drive northwards on the third day of French air strikes, moving deep into the vast territory seized by rebels in April.
France is determined to end Islamist domination of north Mali, which many fear could act as a base for attacks on the West and for links with Al-Qaeda in Yemen, Somalia and North Africa.
France's Defence Minister Jean-Yves Le Drian said French intervention on Friday had prevented rebels driving southward to seize Bamako itself. He said air raids would continue in the coming days.
In Gao, a dusty town on the banks of the Niger river, an extreme form of Sharia law has been imposed.
Residents said French fighters and attack helicopters pounded the airport and rebel positions.
A huge cloud of black smoke rose from the militants' camp in the north of the city.
A Malian rebel spokesman said the French had also bombed targets in the towns of Lere and Douentza.
France has deployed about 550 soldiers to Mali.
The troops are split between Bamako and the town of Mopti, which is 500km north.
State-of-the-art Rafale fighter jets were also dispatched to reinforce "Operation Serval" - named after an African wildcat.
In Bamako, a Reuters cameraman saw more than 100 French troops disembark today from a military cargo plane at the international airport, on the outskirts of the capital.
The city itself was calm, with the sun streaking through the dust enveloping the city as the seasonal Harmattan wind blew from the Sahara.
Some cars drove around with French flags draped from the windows to celebrate the French intervention.