When Western forces helped topple Libya's Moamer Kadhafi they forced hundreds of well-armed Tuareg fighters to flee home to Mali, tipping another fragile African state into chaos, experts say.

And for some observers, the Western powers' role in helping trigger the crisis now gives them a responsibility to help try to end it.

"It must be said and said again that the factor that unleashed all of this is the Western intervention in Libya," said Eric Denece, director of the French Centre for Intelligence Research (CF2R), a think tank.

When Kadhafi's regime fell to a popular uprising backed by NATO warplanes, his Tuareg hired guns fled south across the Sahara to their former homes in Mali and Niger in heavily-armed convoys of off-road pick-up trucks.

"At first, these veterans of the Libyan militia had nothing against Mali, but nature abhors a vacuum, and they needed to find something to do. So they allied with local groups, and now look where we are," he said.

In the past two weeks since a military coup in Mali caused a collapse in government authority, Tuareg rebels have seized the entire north and east of the country, including major towns like Gao and Timbuktu.

Most worryingly for foreign observers, Islamist fighters from the local Ansar Dine group and from Al-Qaeda in the Islamic Maghreb (AQIM), the North African wing of the global extremist network, have been fighting alongside the Tuaregs.

Denece argues that the West, and France in particular, should have been aware of the result of their actions in Libya and acted before now.

"The Malian foreign minister, Soumeylou Boubeye Maiga, came almost every month to Paris for talks at the foreign ministry, the Elysee and the DGSE foreign intelligence service," he told AFP.

"He told them: 'Now that you have intervened and created chaos, what are you doing to help us, the countries in the region? Your intervention spread weapons and fighters across the Sahel. You know we can't fight those people'."

It is difficult to estimate how many fighters in total crossed the desert after the Libyan revolution, but they brought their weapons and training with them and were more than enough to tip the military balance in West Africa.

In Algiers, author and expert Mohamed Mokeddem, put at at least 1,000 the number of Tuareg mercenaries that flooded back into Mali, adding: "What is certain is that the Libyan crisis served AQIM and the Tuareg cause.

"We are seeing the final phase in the creation of a lawless, uncontrollable zone in the Sahel," he told AFP by telephone. "The Malian army can't do anything in this area. They have lost control.

"A good proportion of the Tuareg population, which has been abandoned and marginalised for years, has rallied to the banner of AQIM and local Islamists. They feel like they're taking their revenge," he said.

"The former Kadhafi militiamen are now a force that no-one in the region has the means to oppose. They have reintegrated into the social fabric of North Mali. They have ad hoc alliances with AQIM, Arab tribes and local Islamists."

The Algerian military is the only force in the region with the troops and firepower needed to face down the threat, but it is prevented from operating outside its own national territory by Algeria's constitution, he said.

Denece foresees a nightmare scenario in which the bands roaming the Sahel build ties with fellow Islamist militants in neighbouring areas, such as Boko Haram in Nigeria, and even with the Shebab in far-off Somalia.

Before then, he fears, the West will be obliged to act. "Because of what they did in Libya, they have a real responsibility," he argued.

"If the French, Americans and perhaps the Algerians work together, in three months we could get back to the situation we had before Libya. We'll never liquidate all the gangs, but we could help Mali to take its towns back."