(Adds casualties, fresh quotes, background)

By Tiemoko Diallo

BAMAKO, April 2 (Reuters) - Malian attack helicopters struck at Tuareg rebels in the Sahara on Wednesday, killing several people in an operation meant to stop the insurgents besieging the northeastern town of Kidal, an army source said.

Tuareg raids in Mali's remote north have intensified in recent weeks. Rebels are fighting the army in an apparent effort to maintain control of traditional Saharan smuggling routes.

"The firing lasted all morning. There are vehicles on fire, several dead, without even counting the injured. Several of the rebels fled," a senior military official, who asked not to be named, told Reuters.

He said the decision to strike the rebel positions near Kidal, an ancient desert trading town more than 1,250 km (780 miles) northeast of the capital Bamako, had been taken after the insurgents tried to stop army reinforcements reaching the town.

"They had started laying mines on the road from (the garrison town of) Gao to Kidal to stop all traffic. They were planning an ambush on a military convoy from Gao, so the army had to take the offensive," the military official said.

He said Mi-24 helicopters had opened fire and the army was now waiting for the rebel reaction. The Malian defence ministry said it would give casualty figures later.

The army clashes with Tuareg rebels have taken place in the region where al Qaeda militants are thought to be holding two Austrian tourists seized in Tunisia in February. The violence appears to be unrelated to the kidnapping.

HOSTAGE NEGOTIATIONS

Light-skinned Tuareg nomads launched revolts from Kidal in the 1960s and 1990s demanding greater freedom from a black African-dominated government seated far away in Bamako.

Peace agreements after the 1990s rebellion went some way to addressing Tuareg demands, with former fighters integrated into the army and Tuareg politicians winning more responsibility, but the region remains restive and awash with arms.

Fighting between the army and Tuareg rebels is believed to have complicated efforts to free the two Austrians -- Andrea Kloiber, 43, and Wolfgang Ebner, 51 -- who are thought to be held at an Islamist hideout in the Kidal region.

They disappeared in February while on holiday in Tunisia. Algerian-based al Qaeda in the Islamic Maghreb, in website postings, has set a deadline of April 6 for payment of a ransom and the release of militants held in Algeria and Tunisia.

Tuareg tribesmen have in the past clashed with members of the Salafist Group for Preaching and Combat (GSPC), as al Qaeda in the Islamic Maghreb was previously known, and helped negotiate the release of European tourists kidnapped by them.

Austrian diplomats have been in Mali trying to secure the tourists' release although Mali's government has said there is no concrete evidence the pair are on its territory.

Some reports have suggested they have been moved to northern Mauritania, where the GSPC has been active. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Additional reporting by Mark Heinrich in Vienna; Writing by Nick Tattersall; Editing by Robert Woodward)

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