GAO, Mali: French Defence Minister Jean-Yves Le Drian on Thursday visited French troops in northern Mali, where the military says they have killed 150 Islamist rebels over the past month.

"My first feeling is one of pride," Le Drian said in a speech to around 250 French soldiers on a stop in the northern city of Gao before heading to the capital Bamako.

He praised the troops' professionalism and courage to the point of "giving your life", after a French soldier died on Wednesday, the fourth French death since the launch of the operation on January 11.

The defence ministry said Le Drian started his visit in the Amettetai valley in the Ifoghas mountains, where French and Chadian troops were on Thursday carrying out operations to sweep the valley and other areas of the mountains near the border with Algeria.

The army said French forces have killed more than 150 Islamist rebels since mid-February in Gao and in fighting for the Ametettai valley, where the militants fled after being driven out of their strongholds in the north.

"By dislodging jihadists from their final bastions, you are the bridgeheads in this war... that France has decided to undertake against the terrorist groups still in Mali," Le Drian said in a ministry statement.

"On you, and with our brothers in the Chadian army... rests a large part of the success" of the military intervention, he said.

Le Drian also stressed that "the mission is not over... It is later that we will progressively withdraw to hand over to the African mission under the United Nations."

Mali's military chief General Ibrahim Dembele told journalists in Gao that "more than 70 percent of the work has been accomplished" against the Islamists extremists in the north.

But he added that "residual elements will continue to sow panic, no where is there total security."

Some 4,000 French soldiers are deployed in Mali to fight alongside Malian and West African troops against the Islamist rebels, some of whom are linked to Al-Qaeda in the Islamic Maghreb (AQIM).

France also said Thursday it was carrying out DNA tests to confirm reports from Chad of the killings in Mali of two top Islamist rebels, Mokhtar Belmokhtar and Abdelhamid Abou Zeid.

"We know that there were a fair number of leaders among the several hundred terrorists killed" in recent days, French Foreign Minister Laurent Fabius said on RTL radio.

"Very precise DNA tests must be carried out to determine their identities, which military services are currently doing," he said.

An AQIM source on Monday confirmed the death of its leader in Mali, Abou Zeid, but insisted that Belmokhtar was still alive and fighting.

French President Francois Hollande said Wednesday that "terrorist kingpins have been destroyed" in Mali but did not make it clear if Belmokhtar, the mastermind of the January assault on an Algerian gas plant that left 37 foreign hostages dead, was among them.

Meanwhile a French-Malian citizen arrested in November for allegedly seeking to join Islamist militants in Mali has been expelled to France where he will be held for questioning, a French judicial source said Thursday.

Ibrahim Aziz Ouattara, 25, faces potential terrorism charges in France.