French forces in Mali have been taken by surprise by the fighting strength of the Islamist radicals they are attempting to drive out of the centre of the country, it emerged on Sunday.

Aides to President Francois Hollande admitted the militants were better equipped, armed and trained than they had expected.

"What has struck us markedly is how modern their equipment is and their ability to use it," one said in a reference to the rebels' hit on a French Gazelle helicopter.

The helicopter was forced to make an emergency landing and its pilot, Lieutenant Damien Boiteux, died of his injuries after being shot with a light arm.

The helicopter was brought down during an attack on an Islamist convoy travelling between the towns of Mopti and Sevare in central Mali.

"Initially, they appeared like a rag-tag bunch travelling in their Toyotas with a few weapons," the official added. "But they've shown themselves to be well-equipped, well-armed and well-trained."

The French officials believe the Islamists obtained many of their weapons during last year's unrest, when arms were delivered to rebels fighting to overthrow Libyan dictator Moamer Kadhafi.

"In Libya they picked up modern, sophisticated kit that is a lot more robust and effective than could have been imagined," the source added.

The strength of the Islamists appears to have bolstered France's resolve to limit its intervention to pushing the Islamists back and softening them up for a West African force to re-establish government control over the north of Mali, which has been under the rule of Al-Qaeda-linked groups since April 2012.

"The important word now is 'Africanisation', which means the rapid deployment of an African force. The situation calls for that and it is up to the Africans to restore Mali's integrity," the official said.

For that to happen, the French believe the Islamists will have to be driven out of the principle towns in the north: Kidal, Timbuktu and Gao, which was the target of intensive bombing by French fighter jets on Sunday.