GAO: Chadian troops swarmed into the desert town of Kidal in northeastern Mali on Tuesday, securing the last militant bastion as global players met in Brussels to carve out a path back to stability for the troubled west African nation.

The French defence ministry said 1,800 Chadian troops had entered Kidal to “secure” the Saharan outpost, after days of air strikes in the surrounding mountains where extremist insurgents are believed to be hiding in hillside caves.

“The French are controlling the airport with the back-up of two paratrooper units,” a ministry official said, adding that nearly 4,000 French troops were now on the ground in its former west African colony.

The official said French air strikes had hit 25 targets in recent days, “mainly logistical depots and training centres” in the areas of Aguelhok and Tessalit, near the Algerian border.

French-led forces have driven out the extremist fighters, who had controlled the north for 10 months, from their key strongholds in Gao, Timbuktu and Kidal after sweeping to Mali's aid on January 11 to stop the rebels from advancing on the capital Bamako.

The rebels melted away into the Adrar des Ifoghas massif around Kidal, a craggy mountain landscape honeycombed with caves where they are believed to be holding seven French hostages kidnapped in Mali and Niger in 2011 and 2012.

Dozens of French warplanes have carried out air strikes in recent days on rebel training and logistics centres in the region, near the Algerian border and some 1,500 kilometres northeast of Bamako.

French Foreign Minister Laurent Fabius said Monday the strikes were aimed at blocking the rebels' supply routes to flush them out.

Some 600 French troops based in the fabled city of Timbuktu prepared to withdraw on Tuesday, crossing the Niger River and making their way to Gao before heading to Kidal, a military source told AFP.

On Monday Fabius said France was working to “very quickly” withdraw its forces from Timbuktu and hand the baton over to African troops.

In Brussels, global players met to carve out plans for Mali's future once the 26-day-old offensive draws to an end.

“When a state falls apart it takes time to put it together again, like Humpty Dumpty,” a European Union official said on condition of anonymity. “It's going to take years to achieve the end outcome. But I hope it will only take months to achieve a secure enough environment.”

Mali's Foreign Minister Tieman Coulibaly said the threat in northern Mali “concerns all civilised countries.”

Western powers have raised concerns the zone could become a new breeding ground for terrorists.

On Tuesday three French-Congolese and a Malian were arrested in the Paris region as part of a probe into a network set up to send jihadists to fight in West Africa.

At the top of the immediate political agenda in Brussels will be the dispatch of human rights observers, amid fears of rights abuse and reprisals against light-skinned Tuareg and Arabs who are accused of backing the religious extremists.

Aside from rustling up aid and speeding up plans to deploy a formal United Nations peacekeeping force, the delegations from the UN, African Union and other blocs will also mull how to assist Mali hold elections before July 31.

A little over a year ago the West African nation was considered a stable democracy.

But long-simmering tensions in the north – home to Tuareg desert nomads who have long felt marginalised and have fought several separatist rebellions, as well as a radical Al Qaeda affiliate – erupted and led to the nation's rapid implosion.

Tuareg rebels took up arms again for independence and humiliated the poor and ramshackle Malian army.

Allied with Al Qaeda in the Islamic Maghreb (AQIM) and other radical groups, the Tuareg seized key northern cities and an area as large as Texas in a matter of days.

As political instability lingered in Bamako, there was little to stop the extremists, who sidelined the Tuareg and began running towns under their control with a brutal form of sharia.

The straw that broke the camel's back was a decision by the militants to push into the government-controlled south on January 10 by seizing the town of Konna, prompting France to intervene with a lightning air and ground offensive.