By Richard Valdmanis and Yann Le Guernigou
KONNA, Mali/PARIS Jan 26 (Reuters) - French forces fighting Islamist rebels in Mali seized the airport and a bridge at the rebel bastion of Gao on Saturday, the biggest military success so far for an offensive against al Qaeda-allied insurgents occupying the country's north.
The United States and Europe back the U.N.-mandated Mali operation as a counterstrike against the threat of radical Islamist jihadists using the West African state's inhospitable Sahara desert as a launching pad for international attacks.
In their overnight advance on Gao involving special force troops backed by warplanes and helicopter gunships, the French killed an estimated dozen Islamist fighters without suffering any losses or injuries, the French army said.
The speed of the French action in a two-week-old campaign suggested French and Malian government troops intended to drive aggressively into the north of Mali in the next few days against other Islamist rebel strongholds, such as Timbuktu and Kidal.
There have been 30 French air strikes on militant targets around Gao and Timbuktu in the past 36 hours.
News that the French were at the gates of Gao, the largest northern town held by the Islamists, came as African states struggled to deploy a planned 6,000-strong intervention force in Mali, known as AFISMA, under a U.N. mandate.
French army spokesman Colonel Thierry Burkhard said French forces were still coming under fire from rebels inside Gao.
"At the moment, there are still contacts, some harassment operations by terrorist groups who are firing in the direction of the airport from residences or seeking to blend in with the population," Burkhard told Reuters.
He said both the bridge and airport runway were undamaged.
In Paris, the French defence ministry said that Malian and French troop reinforcements were being brought up and that troops from Chad and Niger, who have experience in desert warfare, would also be flown in shortly to Gao.
To the west, French forces pushed towards Lere, on the road to Timbuktu, without so far encountering resistance.
For two weeks, French jets and helicopter gunships have been harrying the retreating Islamists, attacking their vehicles, command posts and weapons depots. The French action had stymied a sudden Islamist offensive launched in early January that had threatened Bamako, Mali's capital in the south of the country.
Washington and European governments, while providing airlift and intelligence support to the anti-militant offensive, are not planning to send in any combat troops.
FRANCE TAKING THE LEAD
At an African Union summit in Addis Ababa, AU leaders called on the United Nations to provide emergency logistics and funding to allow the African force for Mali to deploy.
AU officials say AFISMA is severely hampered by logistical shortages and needs airlift support, ammunition, telecoms equipment, field hospitals, food and water. It also required training to operate in Mali's desert and arid mountains.
There appeared to be some embarrassment among African ministers and leaders that the continent was having to rely on a former colonial power, France, much criticised for past meddling in Africa, to take the lead in the military campaign in Mali.
Rwandan Foreign Minister Louise Mushikiwabo said France's intervention was "justified".
"If Africa can't do it, somebody else should do it," Mushikiwabo told reporters on the sidelines of the summit.
France, which dispatched its military to Mali at the Bamako government's request, already has 2,500 soldiers on the ground in its former colony.
Around 1,900 African troops, including Chadians, have been deployed to Mali so far. Burkina Faso, Benin, Nigeria, Senegal, Togo, Niger and Chad are providing troops while Burundi and other African nations have pledged to contribute.
While the French and Malians thrust northeast in a two-pronged offensive towards Gao and Timbuktu, Chadian and local forces in neighbouring Niger are preparing a flanking thrust coming up from the south.
A large column of armoured vehicles and hundreds of Chadian troops rolled out of the Niger capital Niamey on Saturday, heading northwards towards the Mali border.
ISLAMISTS "IN RETREAT"
Malian army officers said there had been no direct fighting in recent days as the Islamist insurgents pulled back to avoid deadly French air strikes that have destroyed rebel vehicles, command posts, fuel depots and stores.
"They are all hiding. They are leaving on foot and on motorcycles," Malian Army Captain Faran Keita told Reuters at Konna, about 500 km (310 miles) southeast of Gao.
Konna's capture by the Islamist insurgents on Jan. 10 triggered the sudden French military intervention to aid Mali. Reporters there saw charred rebel pickup trucks that had been blasted by French air strikes. Munitions lay scattered about.
The question remained whether the Islamists would fight to hold Gao and Timbuktu or withdraw further north into the trackless desert wastes and mountain fastnesses of the Sahara.
"We have stopped their offensive, they are in retreat - that is clear. Whether they are in retreat to regroup, what their next intentions are or what has been done to their total strength, that is not clear," a U.S. official in the region, who asked not to be named, said.
On Friday, the Islamists blew up a road bridge on the main road south from Gao to Niger, but military officials from Niger said the Chadian and Nigerien forces could still reach Gao by other routes when they advanced.
U.S. President Barack Obama spoke to French President Francois Hollande by phone on Friday and expressed support for France's military operation in Mali.
At a conference of donors for the Mali operation to be held in Addis Ababa on Jan. 29, the AU is expected to seek hundreds of millions of dollars in logistical support and funding to assist the deployment of the African intervention force. (Additional reporting by James Regan in Paris, Richard Valdmanis in Konna, Mali, Richard Lough and Aaron Maasho in Addis Ababa and David Lewis and Pascal Fletcher in Dakar; Writing by Pascal Fletcher)