Gao: 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, a 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 airplanes, 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 Qaida-linked Islamist militias which occupied the north until the French-led Operation Serval dislodged them in January.

The city, 1,200km 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 6.00am on Sunday.

At 8.00am 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 20km 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.

Around 4,000 French soldiers are engaged in Mali but Paris has said that from the end of April they will begin a staged withdrawal, leaving just a “support force” of 1,000.