FRENCH air strikes have forced Islamist guerrillas to flee towns in northern Mali and Paris secured new international support for military action as the militants struck back, seizing a small western town.

The Islamists said they had made a "tactical retreat" from Timbuktu and other key towns where they have imposed a brutal version of Islamic law for nearly 10 months.

But they struck back in western Mali where they took the small town of Diabaly from the country's weakened army, highlighting the daunting campaign ahead to restore order in the West African nation.

French jets on Monday hit Douentza, 800 kilometres from Bamako, which the Islamists have held since September. But residents said the fighters had left before the warplanes arrived.

In Timbuktu, where inhabitants have been executed or had limbs cut off in some of the worst abuses, the Islamists reportedly fled in anticipation of an attack.

"The mujahideen have left. They are really scared," said one resident in the historic city, where the militants have destroyed centuries-old Muslim monuments.

In Gao, another northern city held by the Islamists, the jihadists were nowhere to be seen after bombing by Rafale warplanes on Sunday, residents there said.

At least 60 insurgents were killed in Sunday's assault, according to residents and a security source.

Mali's Foreign Minister Tieman Hubert Coulibaly said in Paris that he believed more than 100 Islamists had been killed in the four days since France launched operations to stem a guerrilla advance towards Bamako.

A spokesman for the Ansar Dine (Defenders of the Faith) group, Senda Ould Boumama, said the withdrawal was a "tactical retreat" to reduce civilian casualties, in comments published on Mauritanian news website Alakhbar.

A leader of the Movement for Oneness and Jihad in West Africa (MUJAO), vowed revenge.

"France has attacked Islam. We will strike at the heart of France," said Abou Dardar of the al-Qaeda-linked group.

On top of the use of Rafale fighters and helicopter attacks, about 650 French troops are in Mali to halt the Islamist advance, according to the French defence ministry.

While jolted by France's arrival, the insurgents remained on the offensive in areas where French troops were not yet operating. The militants seized Diabaly, some 400km north of Bamako.

"We knew there would be a counter-offensive towards the west," French Defence Minister Jean-Yves Le Drian told BFM Television.

"They have taken Diabaly, which is a small town, after heavy fighting and resistance from the Malian army, which was insufficiently equipped at that exact point."

Le Drian acknowledged that French forces were facing a "difficult" situation in the west, where he said the rebels are well armed.

France's Foreign Minister, Laurent Fabius, hailed the "quasi-unanimous" international backing for the offensive, strongly supported by Mali.

"We cannot simply push them back, we have to chase them away," Coulibaly told French television after meeting Fabius.

"We simply now cannot allow a timeout for these forces to reorganise."

A meeting of the 15-nation UN Security Council on Mali also expressed unanimous "understanding and support" for the military intervention, France's UN ambassador Gerard Araud told reporters.

France and other council countries want to speed up the deployment of a UN-mandated 3300-strong West African intervention force in Mali.

Nigeria, which will lead the force, plans to have 600 troops on the ground "before next week", President Goodluck Jonathan said. Benin, Ghana, Niger, Senegal, Burkina Faso and Togo have also pledged troops.

Britain and Canada have offered military transporters to the French military and the United States said it will share intelligence and provide logistical support.

"I commend France for taking the steps that it has," US Defense Secretary Leon Panetta said.

Algeria said it had closed its 2000km desert border with northern Mali to stop Islamists crossing into the country.

Originally published as Islamists flee French air strikes in Mali