By Camillus Eboh ABUJA, June 16 (Reuters) - Nigeria has put hundreds more suspected Islamist militants on trial in its latest round of mass prosecutions, as authorities step up efforts to tackle a long-running insurgency, Attorney-General Lateef Fagbemi said on Tuesday. Africa's top oil producer is grappling with a 17-year insurgency led by Boko Haram and Islamic State West Africa Province (ISWAP) in the country's northeast.