BAMAKO (AFP) - Islamist fighters in Mali fled their key bases as they were pounded by a fourth day of French airstrikes on Monday, seizing a small town in the government-held south and vowing revenge attacks on France.

As the French offensive was boosted by offers of Western support and logistical aid, Mali's neighbours readied to deploy troops for an intervention that had been plagued by months of torpor before it was kickstarted by France's swift action.

French Foreign Minister Laurent Fabius hailed the "quasi-unanimous" backing from the international community for the offensive, which his Malian counterpart said was aimed at decisively driving out the Islamists.

"We cannot simply push them back, we have to chase them away," Malian Foreign Minister Tieman Hubert Coulibaly told BFMTV after meeting Fabius.

"We simply now cannot allow a timeout for these forces to reorganise." The French flew in with fighter jets on Friday as the Islamists pressed south towards the capital, Bamako, after occupying the vast desert north since April and imposing a brutal form of Islamic law.

While heavily battered by the airstrikes, the insurgents remained on the offensive.

On Monday they seized the town of Diabaly, some 400km north of Bamako, even as Islamist fighters fled many of the cities they held farther north after a weekend of heavy strikes by French warplanes.

"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."

He earlier told journalists that while the Islamists had "retreated" in the east of Mali, French forces were facing a "difficult" situation in the west, where he said the rebels are well armed.

Airstrikes resumed on Monday in Douentza, a town 800km from Bamako that the Islamists have held since September, but by the time the French struck the rebels had already fled.

"They are afraid of the planes. The last 4x4 filled with mujahideen left at about 4pm (12am Singapore time)," said a witness.

In Gao, one of the three main cities in the north seized by the Islamists, the jihadists were nowhere to be seen after heavy bombing by French Rafale fighter planes on Sunday.

"We are free. We haven't seen a single mujahideen here today (Monday). They left the town and the leaders are hiding," said a resident reached by telephone from Bamako, speaking on condition of anonymity.

Earlier, residents and a security source reported at least 60 insurgents killed in the assault.

"I think in the last four days these jihadists have suffered heavy losses with more than 100 deaths," said Foreign Minister Coulibaly.

In the fabled city of Timbuktu, where some of the worst abuses under Islamist control have taken place, their fighters reportedly fled in anticipation of an attack, as no strikes have yet been reported in the desert city.

"The mujahideen have left, they are really scared," said a resident in the town, a Unesco World Heritage site where the jihadists have destroyed centuries-old monuments they considered idolatrous.

The spokesman for rebel group Ansar Dine (Defenders of the Faith), Senda Ould Boumama, said the withdrawal was merely a "tactical retreat", in comments published on Mauritanian news website Alakhbar.

"Our movement's fighters have retreated from the towns and positions they occupied...with the goal of limiting damage among unarmed civilian populations," he said.

Algeria's foreign ministry confirmed Monday it had closed its 2,000km desert border with northern Mali.

Previously opposed to military intervention, the regional military heavyweight has thrown support behind the offensive and is allowing French Rafale fighter jets to use its airspace.

Media reports have said France is deploying about 500 troops in Mali.

A convoy of some 30 French armoured tanks and military vehicles escorted by a helicopter crossed into Mali from Ivory Coast on Monday, cheered on by villagers, residents said.

A leader of one of the Islamist groups, the Movement for Oneness and Jihad in West Africa (MUJAO), vowed revenge against France, which has stepped up security on home soil in fear of reprisal attacks.

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

Another MUJAO leader, Omar Ould Hamaha, nicknamed "Redbeard", warned on radio Europe 1 that France had "opened the doors of hell" with its intervention and faced a situation "worse than Iraq, Afghanistan or Somalia".

At the request of Paris, the UN Security Council met on Monday to discuss the conflict. The council is seeking speedy implementation of an earlier resolution that authorises the deployment of a 3,300-strong West African intervention force.

Nigeria, which will lead the force, plans to have 600 troops on the ground "before next week", President Goodluck Jonathan said on Monday.

Niger, Senegal, Burkina Faso and Togo have each pledged 500 troops, while Benin and Ghana have also said they will contribute.

The Islamists' advance in Mali has raised fears the country could provide a safe haven for Al-Qaeda-linked extremists, posing a threat to the region as well as Europe and beyond.

France's intervention has been backed by the European Union, Nato and the US, while Britain and Canada will provide logistical support in the form of transport planes.