Caught off guard by France's fierce offensive against their bases, Islamists occupying northern Mali have fled into the vast desert to regroup and plot their next move, experts said Tuesday.

Masters of the harsh desert terrain, experts of survival and well-armed, the jihadists' retreat should not be mistaken for a sign of defeat, analysts said.

Terrorism expert Jean-Marc Brisard said the Islamists' evacuation of their bases after heavy air strikes by French fighter jets was an obvious "tactical retreat."

He said the insurgents were merely taking advantage of "the immensity of the desert" of northern Mali, whose main towns they have controlled since overpowering a weakened Malian army in April.

"To our knowledge they have retreated into the mountainous regions around Kidal. It will be necessary to quickly go on the ground to dislodge them," he said.

Kidal, along with Gao and Timbuktu, has been one of the key centres of the Islamist-conquered Malian territory, a vast expanse of hostile, semi-arid terrain more than double the size of France.

The international community has long feared the zone could become a new breeding ground for terrorism, and a push by the insurgents towards Bamako prompted France to intervene on Friday.

Backed by the remnants of the battered Malian army, the former colonial power swept in with Rafale fighter jets and hundreds of troops in a bid to drive back the Islamists.

Allied and sometimes overlapping, the rebel groups Al-Qaeda in the Islamic Maghreb (AQIM), Ansar Dine (Defenders of Faith) and the Movement for Oneness and Jihad in West Africa (MUJAO), have implemented brutal sharia law in towns under their control.

Whippings, amputations and sometimes executions of transgressors have been accompanied by the destruction of centuries-old monuments which the mujahideen consider "idolatrous".

French warplanes sent the Islamists fleeing from Kidal and Gao as well as the more central town of Douentza, destroying weapons and fuel stockpiles and other bases in five days of airstrikes.

But elsewhere the jihadists remained on the offensive, on Monday seizing the small town of Diabaly 400 kilometres north of Bamako, which was targeted several hours later by French forces.

In fear of the security threat posed by the fleeing extremists, Algeria has closed its 2,000 kilometre-long border with Mali and Mauritania has deployed troops to patrol its frontier along the west of Mali.

Meanwhile West African nations Nigeria, Benin, Burkina Faso, Senegal, Ghana, Togo and Guinea, are preparing to deploy a UN-backed force of some 3,300 men, with the first Nigerian troops set to arrive in Mali by Wednesday.

This intervention force has been fast-tracked after months of concern over the training and capability of the African troops, and some experts warn it could still be a while before they are operational faced with hardened fighters accustomed to harsh desert conditions.

"They have dispersed to make the African offensive a lot more difficult on the ground. They are spreading out to draw us into a guerrilla situation. They can prolong the conflict indefinitely," said Brisard.

Alaya Allani, a Tunisian expert on Islamist movements, agrees.

"The jihadists are in it for the long-haul. They are comfortable in this situation: the vast desert, a difficult terrain, a precarious security situation."

"They will make a tactical withdrawal to get their breath back, but neither the Malian army or the French can crush them."

Malian expert Moussa Tounkara said the withdrawal from the main Malian towns was temporary, and warned the Islamists were likely to strike back with targeted attacks elsewhere.

Islamist leaders in Mali, outraged at the French offensive, have vowed to strike "at the heart" of France, including its interests in Africa.

Concern is also high over the fate of seven French hostages kidnapped in the Sahel over the past two years.

Pierre Camatte, a French hostage held for three months in 2009 before being rescued, said the danger for his countrymen was "at its highest point", while agreeing the French intervention was necessary.

"Today there are real fanatics who consider themselves entrusted with a divine mission to combat heathens, notably the French."