TIMBUKTU/GAO, Mali: French troops began to withdraw from Timbuktu Thursday after securing the fabled city as they ramped up their mission in another northern Mali city, searching for Islamic extremists who may be mixing among the locals.

French military spokesman Col. Thierry Burkhard said Thursday that the operation to secure Gao was still under way, nearly two weeks after French and Malian troops moved into the area. New clashes nearby raised questions about how solid a hold the French military has on the strategic area.

There is a risk of “residual presence” of terrorists mixed among the population, Burkhard said from Paris.

Four Malian troops were killed by a land mine in territory reclaimed from Islamist rebels, police said Thursday.

The deadly explosion Wednesday between the northern towns of Douentza and Gao came six days after a similar blast in the same area killed two Malian soldiers, underlining the danger the Islamist fighters still pose despite fleeing the towns.

“A Malian army vehicle was blown up by a mine placed by the Islamist criminals,” a paramilitary police officer told AFP.

France launched a military operation in Mali on Jan. 11 to help the Malian government restore control. Islamic extremists linked to Al-Qaeda had imposed severe rule in northern Mali then started pushing toward the capital last month.

The Movement for Oneness and Jihad in West Africa (MUJAO), one of the Islamist groups that seized control of northern Mali for 10 months, said it had “created a new combat zone” and claimed two recent attacks on the road to Gao, the largest city in the north.

With fears of a prolonged insurgency, Paris is keen to hand over the military burden of an operation the Defense Ministry said had already cost France 70 million euros ($95 million), with the figure rising by 2.7 million euros per day.

French troops have also begun to draw out of Timbuktu, after greater successes in securing the desert city.

Soldiers in fatigues could be seen pushing an artillery cannon onto a barge crossing the Niger River, located on the southern perimeter of Timbuktu. France has commandeered the river crossing, and Thursday small convoys of military vehicles were lining up, waiting for the barge, including armored cars, trucks covered with camouflage-colored tarps and vehicles loaded with supplies such as cartons of bottled water.

While the population of Timbuktu is anxious, worrying that the departure of French troops will open the door for the Islamists to return, French military officials said they had fulfilled their mission here.

“We have succeeded in handing over the majority of our responsibilities to the Malian army and now she will assume our duties. But we will not leave the city of Timbuktu completely,” said Capt. Franck, an official with the French operation codenamed Serval, after a sub-Saharan wildcat. He gave only his first name in keeping with military protocol.

He said some French forces would stay because “once we are gone, these people will come back in order to trouble the population. At the same time, we can’t stay indefinitely.”

French president Francois Hollande has said France could begin withdrawing its 4,000 troops from Mali as early as March.

France now has as many soldiers in Mali as it had at the peak of its deployment in Afghanistan in 2010.

Foreign Minister Laurent Fabius reiterated that stance Thursday, saying the administration was sticking to its schedule and emphasizing the need for political as well as military action.

“Our objective cannot be achieved with arms only,” Fabius said in an interview on French television BFM.

Fabius said France had carefully studied foreign interventions elsewhere in the world before undertaking the Mali mission. “It’s not Afghanistan, it’s not Somalia, but there are nonetheless lessons to be learned.”

Still, for residents of Timbuktu, which was subjected to 10 months of often-brutal Shariah rule, the departure of the troops is premature.

“It really worries me to see the French military leave right away,” said Abdel Kader Konta, the village chief of Korioume, the locality from which the troops were embarking onto the barge. “We think it’s too early for them to leave because the Islamists have not fully quit the city. Some of the Islamists have simply shaved their beards and blended into the population. Before the French leave, they should assure themselves that security has been restored.”

Curious onlookers gathered near the river crossing to watch the French departure, which is expected to be phased over five days. Several had long faces, despondent with worry.

“People think that the Islamists have left. But we think they are still here,” said fisherman Baba Ali Sampana, who had stopped to watch their departure, standing next to his fishing canoe. “The French military should not leave right now.”

Further north, French troops are in control of the Kidal airport, while the city and surroundings are patrolled by some of the 1,800 Chadian troops taking part in the operation.

The Economic Community of West African States is slowly deploying some 6,000 troops to join the French forces.