* Defence Ministry says army drove off Islamist attack
* Clash took place close to strategic town of Mopti
* AU chief says NATO should join Mali force
Mali's army repelled an attack on its advance positions by heavily armed Islamist groups moving southward, the Defence Ministry said on Tuesday, in the first clashes since militants seized the country's north in April.
In a communique, the ministry said government forces had clashed with fighters from al Qaeda's north African wing AQIM as well as the Ansar Dine and MUJWA Islamist movements late on Monday close to the town of Mopti, 450 km (280 miles) northeast of the capital Bamako.
Convoys of pick up trucks carrying Islamist fighters had been reported moving southwards towards Mopti, which has a large military barracks and airport.
One of the mostly northerly towns still controlled by the government, Mopti lies at the bottleneck between Mali's arid north and the more populous south.
"The armed forces have driven off this attempted attack," said the ministry statement, read out on Malian radio and television.
Military sources had earlier said the army had simply fired warning shots from heavy artillery from its advanced positions to dissuade the rebels from advancing further.
The capture of the northern two-thirds of the arid West African nation by the loose coalition of Islamist groups has sown fears among Western and regional powers that Mali could become a haven for radicals to plot international attacks.
It was not immediately clear whether the rebels' southward push toward Mopti was an attempt to take the town or simply a show of force ahead of peace talks this week. Rebel spokesmen have declined to comment on their tactics
The peace talks between some of the northern rebel groups and the government are scheduled for Thursday in Ouagadougou, the capital of neighbouring Burkina Faso, but the growing military tension could derail them.
Ansar Dine, one of the main rebel factions, announced last week it had ended a ceasefire because of international plans to deploy an African-led force to drive the al Qaeda-linked fighters from Mali's north.
A spokesman for Ansar Dine, whose leadership sprang from Mali's Tuareg MNLA rebel group and is considered less militant than AQIM and MUJWA, refused to comment on whether it had clashed with government forces.
The head of the African Union, Benin President Boni Yayi, said the seriousness of the situation in Mali required Western powers to join the African-led operation, which received the green light from the U.N. Security Council last month.
"NATO should join up with our African forces," he said during a visit to Canada. A military operation is not expected before September.
Once an example of democracy and development in turbulent West Africa, Mali was plunged into crisis by a March 2012 coup which allowed Tuareg rebels to seize the country's north, demanding an independent homeland. Islamists who initially fought alongside them soon came to dominate the rebellion.
There appears scant chance of a breakthrough in this week's talks. Ansar Dine has insisted it will not drop demands for Islamic law and autonomy for north Mali, while President Dioncounda Traore has insisted he will not compromise over Mali's secular state and territorial integrity.