GAO, Mali: A French force of 1,000 soldiers has begun a sweep of a river valley thought to be a logistics base for armed Islamists near the Malian city of Gao, an AFP journalist accompanying the mission said.

Operation Gustav, one of France's largest actions since its intervention against insurgents in January, will involve dozens of tanks, helicopters, drones and aircraft, said General Bernard Barrera, commander of the French land forces in Mali.

Gao, the largest city in northeastern Mali, was a stronghold of the Movement for Oneness and Jihad in West Africa (MUJAO), one of the Al Qaeda-linked Islamist militias which occupied the north until the French-led Operation Serval dislodged them in January.

The city, 1,200 kilometres (750 miles) from the capital Bamako, suffered the first suicide bombings in Mali's history in February and has been the scene of violent clashes between the French-Malian forces and jihadists involving seven deaths over the last two weeks.

No Islamist fighters were encountered on the first day of Operation Gustav, launched at dawn on Sunday, but troops neutralised around 340 artillery shells and high-calibre rockets found stashed under acacia trees in ravines.

“We surrounded the valley north of Gao, which we believe serves as a logistics base for jihadist groups, and we began to search methodically,” said Barrera, who is based in Gao but joined the troops late on Sunday afternoon.

All access points to the valley, north of Gao, were shut off and its ridges secured at 6:00 am (local and GMT) on Sunday. At 8:00 am France's 3rd Mechanised Brigade began the excavation of a thick forest where military intelligence suspects a jihadist base may be hidden.

French soldiers will spend the coming days combing the 20-kilometre (12-mile) valley with the help of Malian soldiers and police officers who will first go into the nomad camps and mud houses which line the dry river basin.

“This is the fourth wadi we have gone into in the Gao region. There will no doubt be other such operations but perhaps not to the same extent,” Barrera said.

Ethnic Tuareg rebels seized the country's vast arid north in the chaos following a coup in Bamako on March 2012 before losing control to well-armed Islamists.

A French-led intervention quickly drove insurgents from most of their northern strongholds, but significant pockets of resistance remain in Gao, as well as in the fabled desert city of Timbuktu and the Ifoghas mountains.

France is to start withdrawing its 4,000 troops at the end of April, and plans to leave a “support force” of 1,000 soldiers after elections promised for July.