GAO, Mali: French and Chadian troops have pushed to the far northeast of Mali, up to the mountain range where Islamist fighters are thought to be holed up with seven French hostages.

The joint force arrived late Thursday at the town of Aguelhok, 160 kilometres (100 miles) north of the town of Kidal, near Mali's border with Algeria, Malian sources said.

Nearly a month after France sent in the first fighter jets and attack helicopters, it has largely driven the rebels into remote mountains in the far northeast. But the threat from the rebels is still real.

"French and Chadian soldiers have left Kidal and are currently patrolling in Aguelhok," Malian Captain Aliou Toure told AFP.

"The French and Chadian soldiers left in strength by road," said an official with the administration in Kidal. "They arrived at Aguelhok and are then heading for Tessalit." Tessalit lies even closer to the Algerian border.

Both towns had been targeted with repeated French air strikes over the past few days aimed at knocking out Islamist bases, French military spokesman Colonel Thierry Burkhard said.

The two towns lie in the Adrar des Ifoghas massif, in the far northeast, a craggy mountain landscape honeycombed with caves, where the insurgents are believed to have fled with seven French hostages.

One of the Islamist groups, the Movement for Oneness and Jihad in West Africa (MUJAO), said in a message to AFP Thursday that it had "created a new combat zone" by organising attacks on military convoys and placing landmines.

A landmine blast on Wednesday between the northern towns of Douentza and Gao killed four civilians returning from market, an officer with Mali's paramilitary police said.

That explosion came after a similar blast in the same area on January 31 claimed the lives of two Malian soldiers.

"MUJAO is behind the explosion of two Malian army cars," the group's spokesman Abu Walid Sahraoui said in a text message to AFP.

He called on Malians to stay away from main roads, which he said had been heavily mined.

French-led forces continue to come under attack in reclaimed territory, including rocket fire directed at them Tuesday in Gao, the largest city in the north.

The shift to guerrilla tactics by the Al Qaeda-linked groups, which for 10 months occupied Mali's vast desert north, came as France sought to hand over to UN peacekeepers.

France had moved in as the rebels pushed south, sparking fears that they might try to take the capital, Bamako.

Large numbers of troops from France, Mali and Niger have been patrolling Gao and French helicopters have been monitoring the road between Gao and Douentza, 400 kilometres (250 miles) to the southwest.

In New York, UN leader Ban Ki-moon expressed concern at the risk of a guerrilla fightback.

"I think these military operations so far have been effective and successful," Ban said on Thursday.

"All these jihadis and armed groups and terrorist elements -- seemingly they have fled," he added. "Our concern is that they may come back."

Paris is keen to hand over the military burden of an operation the defence ministry said had already cost France 70 million euros ($95 million), with the figure rising by 2.7 million euros every day.

France now has 4,000 troops in Mali, as many as it had in Afghanistan at the peak of its deployment in 2010.

After announcing plans to start withdrawing its soldiers in March, France on Wednesday called for a United Nations peacekeeping force to take over.

But Ban warned it would take weeks for the UN Security Council to decide the international community's next move and UN officials stressed that Mali's interim government had yet to accept a UN force.

The Economic Community of West African States (ECOWAS) is slowly deploying 6,000 troops in Mali, joined by another 2,000 from Chad.

In Cairo, the Organisation of Islamic Cooperation backed efforts to help Mali "regain its territorial integrity", in what appeared to be an endorsement of France's military intervention.