* France says about a dozen "terrorists" killed in operation
* Gao action reveals rapid French advance against Islamists
* AQIM leader vows resistance to "new Crusader aggression"
* African force struggles with logistics, seeks help (Adds Panetta, Le Drian call)
By Richard Valdmanis and Yann Le Guernigou
KONNA, Mali/PARIS Jan 26 (Reuters) - French and Malian forces fighting Islamist rebels took control on Saturday of the rebel bastion of Gao, the biggest military success so far in 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 Islamist jihadists using the West African state's inhospitable Sahara desert as a launching pad for international attacks.
In an overnight assault on Gao backed by French warplanes and helicopters, French special forces seized the town's airport and a key bridge over the River Niger, killing an estimated dozen Islamist fighters without suffering any losses or injuries, the French army said.
"The Malian army and the French control Gao today," Malian army spokesman Lieutenant Diaran Kone told Reuters.
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 and Malian troops were at Gao, the largest northern town held by the Islamists, came as African states struggled to deploy their intervention force in Mali, known as AFISMA, under a U.N. mandate.
Regional army chiefs said on Saturday that a total of 7,700 African soldiers would be dispatched, up from 5,700.
Liberia, Guinea-Bissau, Burundi, Guinea and Uganda are due to join the mission, but it was not clear if progress had been made at meetings in Abidjan or Addis Ababa to overcome gaps in transport, equipment and financing.
French army spokesman Colonel Thierry Burkhard said French forces had come under fire from rebel fighters inside Gao, but that both the bridge and airport runway were undamaged.
In Paris, the French Defence Ministry said Malian and French troop reinforcements were brought in and that soldiers from Chad and Niger, who have experience in desert warfare, were also flown in.
Those Malian and regional troops would have the task of securing Gao and its surrounding area, the ministry said.
To the west, French forces recaptured Lere, on the road to Timbuktu, and were advancing, a Malian military source said, asking not be named.
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.
Reacting to the French-led offensive, one of the leaders of the alliance of Islamist groups occupying Mali's north promised resistance to what he called the "new Crusader aggression", in comments published by Al Jazeera's Arabic website.
Yahya Abu Al-Hamman, leader in the Sahel of al Qaeda's North African wing AQIM, which along with Malian militant group Ansar Dine and AQIM splinter MUJWA occupies Mali's north, said a "Jihadist Islamist emirate" would be created in the territory.
Washington and European governments, while providing airlift and intelligence support to the anti-militant offensive in Mali, are not planning to send in any combat troops.
U.S. Defense Secretary Leon Panetta in a call on Saturday with his French counterpart, Jean-Yves Le Drian, commended France's "strong leadership" in the effort and said the U.S. Africa Command would support the French military by conducting aerial refueling missions.
They also discussed plans for the United States to transport troops from African nations, including Chad and Togo, to support the international effort in Mali, Pentagon spokesman George Little said.
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.
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.
About 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.
FRANCE: 'LOT OF WORK' AHEAD
Malian army officers said the Islamist insurgents had pulled back to avoid deadly French air strikes.
"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. 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 farther north into the trackless desert wastes and mountain fastnesses of the Sahara.
"France can expel rebels from certain of the key towns, but it cannot occupy and control the entire north Mali. North Mali is the size of France," Jakkie Cilliers, executive director of the Pretoria-based Institute for Security Studies, told Reuters.
On a visit to Chile, French Prime Minister Jean-Marc Ayrault admitted French forces in Mali still faced "a lot of work".
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.
The AU is expected to seek hundreds of millions of dollars in logistical support and funding at a conference of donors for the Mali operation to be held in Addis Ababa on Jan. 29. (Additional reporting by James Regan in Paris, Tiemoko Diallo in Bamako, Richard Lough and Aaron Maasho in Addis Ababa, Ange Aboa in Abidjan, Alexandra Ulmer in Santiago, Sami Aboudi in Dubai, David Lewis and Pascal Fletcher in Dakar, and David Alexander in Washington; Writing by Pascal Fletcher; Editing by David Lewis, Jason Webb and Peter Cooney)