ABUJA (Reuters) - Armed men killed 15 people in raids in Nigeria's Sokoto state, government officials said, despite a military effort to stamp out a tide of violence in the northwest.

       Sokoto state Gov. Aminu Waziri Tambuwal described the attackers as "bandits graduating into terrorists".

       The attacks took place from Sunday night into the early hours of Monday morning, he said in a statement. Thirteen people were killed in Illela, a town bordering Niger republic, and two were killed in Goronyo, 76 km (50 miles) east of the state capital.

       Last month, gunmen killed at least 43 people in an attack in the state.

       Groups of armed men operating for profit have killed or kidnapped hundreds of people across northwestern Nigeria this year. There is a telecommunications blackout in place across large swathes of the northwest to aid an armed forces crackdown.

       President Muhammadu Buhari condemned the attack and said the military is deploying advanced technology to track down attackers.

       "This persistent and unprovoked violence against unarmed civilians must be met with fierce response by the government," Buhari said in a statement.

       (Reporting By Tife Owolabi and Felix Onuah, writing by Libby George, editing by Angus MacSwan)