As France begins withdrawing its troops from Mali, a top US defence official has said a UN-mandated African force was "incapable" of taking over the battle against Islamist extremists.

Paris, which sent 4,000 troops to Mali in January to block a feared advance on the capital Bamako by Islamist fighters, said Tuesday it had pulled out its first batch of soldiers preparing to hand over to an African force of 6,300 in the coming weeks.

However, a senior Pentagon official told a congressional hearing in Washington Tuesday that troops from the Economic Community of West African States were not up to the task.

"Right now, the ECOWAS force isn't capable at all. What you saw there, it is a completely incapable force. That has to change," said Michael Sheehan, assistant secretary of defence for special operations.

At the same time, he praised the French troops which "very rapidly" pushed Al-Qaeda's north African branch "back across the Niger river and took control of the major cities" in northern Mali, he said.

However, he added that much of the Al-Qaeda leadership had escaped.

"They haven't been killed or captured, but they (the French forces) have disrupted this very threatening sanctuary."

France says it plans to gradually pull its soldiers out of its former colony, but will leave a permanent 1,000-strong force to fight terrorism.

The French military's chief of staff said around 100 soldiers had been withdrawn and sent to Paphos in Cyprus on Monday, where they will spend three days in a hotel before heading back to France.

They belonged to parachute units of the army that had been deployed in the Tessalit region of northeast Mali, where heavy fighting against Islamists took place, said Thierry Burkhard, chief of staff spokesman.

The Malian military -- poorly paid, ill-equipped and badly organised -- fell apart last year in the face of an uprising by ethnic Tuareg rebels who seized the vast arid north in chaos following a March coup, before losing control to well-armed Islamists.

The extremists terrorised locals with amputations and executions performed under their brutal interpretation of sharia Islamic law, drawing global condemnation and prompting France's January intervention.

While French-led troops have inflicted severe losses on the Islamists, soldiers are still battling significant pockets of resistance in Gao, as well as in the fabled desert city of Timbuktu.

France this weekend launched one of its largest actions since its intervention -- an offensive that swept a valley thought to be a logistics base for Al-Qaeda-linked Islamists near Gao.

In this region, the Movement for Oneness and Jihad in West Africa (MUJAO) -- the most active Islamist rebel group on the ground -- still has the support of some of the population.

But the next phase involving the UN-backed West African troops should have a realistic mission, Sheehan said, in which the peacekeepers would be expected to secure cities but not hunt down militants in remote areas of northern Mali.

"That type of force should be able to take back those cities and allow the French to focus its smaller force in the future on high value targets," he said.

According to a French intelligence expert, the Islamist rebels' ability to inflict severe damage remains limited.

"In three months, the amount of terrorist activity has been very low, if nearly non-existent," said Eric Denece, head of the French Centre for Intelligence Studies.

He pointed out that out of 1,500 to 2,000 known extremists, more than 600 were thought to have been killed.