French and Malian troops have captured another northern town in their advance on the Islamist stronghold of Gao, as the insurgents bombed a key bridge to thwart a new front planned in the east.

But as the French-led assault against the radical Islamists controlling northern Mali entered its third week, aid workers warned about the growing food crisis for civilians in the vast semi-arid zone.

"At present, Malian and French soldiers are in Hombori," said a teacher in the town, which lies 920 kilometres (575 miles) north of the capital Bamako and 200 kilometres west of Gao.

"There are no longer any Islamists on the ground."

A Malian security source said the troops would press on to Gao, one of the three major northern towns along with Kidal and Timbuktu, where the Al Qaeda-linked Islamists have imposed a brutal form of sharia for 10 months.

To the west, the source added, French-led forces who had recaptured the town of Diabaly on Monday were pushing towards the town of Lere with the aim of "taking control of Timbuktu" further north.

Gao, Timbuktu and Kidal were seized by an alliance of Tuareg rebels -- who wanted to declare the independence of the north -- and hardline Islamist groups in April last year.

The Islamists hijacked the rebellion and sidelined the Tuaregs to implement radical Islamic laws, flogging, stoning and executing transgressors, forbidding music and television and forcing women to wear veils.

France swept to the aid of the weakened Malian army on January 11 as the Islamists pushed south towards the capital Bamako. French airstrikes on Gao and Timbuktu forced the Islamists to flee.

On Friday however, the insurgents blew up a key bridge at Tassiga in Mali, which would have given African troops gathering in Niger to the south easy access to Gao.

More than than 2,000 Chadian soldiers have arrived in Niger, joining 500 soldiers from the country's army, to tackle the Islamist forces by opening a second front against them.

"The Islamists dynamited the Tassiga bridge. No one can pass to Niger or come to Gao," said the owner of a transport business, Abdou Maiga. A security source from Niger confirmed the strike.

Mounting concerns on rights abuses

Aid groups warned of rising food insecurity as fighting escalated in the drought-wracked Sahel.

A statement from French aid group Action Against Hunger (ACF) raised fears "that an armed ground intervention from Niger will cut the last access route to supply basic goods (food and medicine) to people in the region".

And a day after rights groups expressed concern about reports of executions of ethnic Tuaregs and Arabs carried out by soldiers of Mali's army, fresh evidence emerged of killings carried out by the Islamists.

UNHCR spokeswoman Melissa Fleming said one person who had fled the city of Gao described seeing "a woman being executed summarily for refusing to show the contents of her bag to a rebel fighter as she tried to board a bus".

Meanwhile, US President Barack Obama backed France's military intervention during a telephone conversation with French President Francois Hollande.

"The president expressed his support for France's leadership of the international community's efforts to deny terrorists a safe haven in Mali," said a White House statement.

In the Ethiopian capital Addis Ababa, the African Union said it would be calling on its members to bolster the strength of AFISMA, the African-led force for Mali.

The AU would also seek support from the United Nations for the operation, in the form of transport, medicine and field hospitals, peace and security commissioner Ramtane Lamamra told reporters.

West African defence chiefs plan to review progress on the slow deployment of their forces to Mali at an emergency meeting in Ivory Coast on Saturday.

Although the Economic Community of West African States has pledged more than 4,500 soldiers to help Mali retake its Islamist-occupied north, their deployment has been delayed by financing and logistical problems.

France has asked several Western countries and others to provide logistical support such as planes to allow aerial refuelling, sources close to Defence Minister Jean-Yves Le Drian said.

Britain, which has already contributed two C-17 transport planes to airlift military equipment, said it had sent a surveillance plane to aid operations.