French forces have launched one their biggest offensives against militants in northern Mali, officials have said.About 1,000 troops are sweeping through a river valley believed to be a logistics base for the armed Islamists near Gao, AFP news agency reports.This is said to be the last major French-led operation before France starts reducing its military presence.The militants have been driven out of northern cities and towns since France intervened militarily in January.However, the Islamists have carried out several suicide attacks in Gao, about 1,200 kilometres (750 miles) north of the capital Bamako, and the ancient city of Timbuktu.Nomad campsThe BBC's West Africa correspondent Thomas Fessy says the French offensive, called "Operation Gustav", is intended to clear the Gao region of any militants still hiding there and to prevent further attacks on the city.France wants to search as much of the remote region as it can for militant hideouts before it starts scaling down its troops from the end of this month as this the kind of operation that African forces may not be able to carry out once they take over, he says.No Islamist fighters were encountered on the first day of the operation, launched at dawn on Sunday, AFP quotes a journalist travelling with the troops as reporting.The French forces neutralised around 340 artillery shells and high-calibre rockets found stashed under acacia trees in ravines, it reports.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."We surrounded the valley north of Gao, which we believe serves as a logistics base for jihadist groups, and we began to search methodically," AFP quotes French land forces commander General Bernard Barrera as saying."This is the fourth wadi [valley] we have gone into in the Gao region. There will no doubt be other such operations but perhaps not to the same extent."France plans to start withdrawing the first of its 4,000 troops later this month, and hopes to have only 1,000 soldiers in Mali by the end of the year.The regional African force in Mali currently numbers about 6,300 soldiers.UN chief Ban Ki-moon has suggested that a 11,000-strong UN peace force, made up of African troops, be deployed in Mali, once France reduces its presence.Mr Ban also called for the creation of a second force to fight militants.Remaining French troops could be part of this force, correspondents say.