BAMAKO (AFP) - French-led troops on Saturday seized the airport and a key bridge serving the Islamist stronghold of Gao in a major boost to a 16-day-old offensive to rout Al Qaeda-linked rebels from Mali's sprawling desert north.

The stunning advance came as the extremist Muslim group controlling Gao since June said it was ready for talks to free a 61-year-old French hostage kidnapped in November.

In a parallel movement, Chadian troops deployed in Mali's eastern neighbour Niger started rolling towards the border to join a contingent of Niger soldiers as part of African efforts to boost the French-led offensive.

"They are a very big contingent and they have tanks and four-wheel drives with machineguns," a Niger security source said.

It was not clear whether they were set to cross the border, which lies only 100 km from Gao.

France on Saturday confirmed the capture of the airport and the Wanbary bridge at Gao but said fighting was continuing in Gao itself.

The airport is located about six kilometres east of Gao, while the bridge lies at the southern entrance to the town, held by the Al Qaeda-linked Movement for Oneness and Jihad in West Africa (MUJAO).

Sources said earlier that the Islamists had left Gao in the wake of the French-led military offensive on Jan 11 to stop a triad of Al Qaeda-linked groups from pushing southward from their northern bastions towards Bamako.

An alliance of Tuareg rebels who wanted to declare an independent homeland in the north and hardline Islamist groups seized the northern towns of Gao, Timbuktu and Kidal in April last year.

The Islamist groups include MUJAO, Ansar Dine, a homegrown Islamist group, and Al-Qaeda in the Maghreb, of which MUJAO is an offshoot.

The Islamists then sidelined the Tuaregs to implement their own Islamic agenda. Their harsh interpretation of sharia law has seen transgressors flogged, stoned and executed, and they have forbidden music and television and forced women to wear veils.

The MUJAO said it was ready for negotiations to release Mr Gilberto Rodriguez Leal, a French national of Portuguese origin who was kidnapped in western Mali.

"The MUJAO is ready to negotiate the release of Gilberto," said spokesman Walid Abu Sarhaoui. "We Muslims can come to an understanding on the issue of war," he added, without elaborating.

West African defence chiefs meanwhile met to review the slow deployment of regional forces to bolster the French-led offensive against Islamists at an emergency meeting in Ivory Coast's main city Abidjan.

Although the Economic Community of West African States (ECOWAS) regional bloc has pledged more than 4,500 soldiers, their deployment has been delayed by financing and logistical problems.

Chad, which neighbours Mali and is not an ECOWAS member, has promised a total of 2,000 additional troops. They were sent to Niger to join 500 local troops to open a new front against the Islamists.

The Abidjan talks will determine exactly how many troops each country in the 15-nation bloc is willing to pledge but "particularly commit to deploying troops as quickly as possible," said Ivory Coast Defence Minister Paul Koffi Koffi.

The African Union said it would urge members to bolster the African force and seek support from the United Nations for the operation in the form of transport, medicine and field hospitals.

While a fraction of the African forces have arrived in Bamako and are slowly deploying elsewhere, the French and Malian forces have done all the fighting so far.

France has already deployed 2,300 troops to Mali and defence officials acknowledge the force will exceed the initially set upper limit of 2,500.

On Friday, the French and Malian forces captured Hombori, another northern town, in their advance on Gao.

To the centre, French-led forces who on Monday had recaptured the town of Diabaly were pushing northeast towards the town of Lere with the aim of taking control of Timbuktu, still further north.

Aid agencies have expressed increasing concern about the growing food crisis for civilians in the vast semi-arid north of Mali and the drought-stricken Sahel as a whole.

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.