Dozens of Islamist militants have drowned in a river while fleeing ground and aerial assaults in the north-eastern Nigerian state of Borno, security sources say.Troops attacked villages on the edge of Sambisa Forest where Boko Haram fighters have their hideouts.The jihadists are reported to have abandoned their positions, rushing into a river swollen by heavy rain.Local militias who fight alongside government troops said they pulled more than 100 bodies from the water and buried them nearby.Tens of thousands have been killed and about two million displaced by the Islamist insurgency which began in 2009.