MAIDUGURI, Nigeria (Reuters) - Islamist militants killed seven Nigerian soldiers, including a commanding officer and a lieutenant, in northern Nigeria's Borno state, a military source and two residents told Reuters on Friday.

       The militants, who the sources said were with Islamic State West Africa Province (ISWAP), attacked a military outpost in Rann, in Kala Balge local government area of Borno state, in three trucks at roughly 1:30 am, firing sporadically for an hour. When the shooting stopped, the soldiers went into the town and were killed in an ambush, the sources said.

       "Those that had hidden inside the town ambushed and killed the captain, lieutenant and five others soldiers," the military source said.

       A military spokesperson did not reply to calls or text messages seeking comment.

       The Nigerian military has fought an Islamist insurgency in northeastern Nigeria for more than 12 years, launched by Boko Haram and joined later by its offshoot ISWAP, a regional affiliate of Islamic State. The United Nations estimates that the conflict had killed some 350,000 people by the end of 2020.

       Last month, ISWAP killed a Nigerian army general and three soldiers in Borno state.

       (Reporting by Maiduguri newsroom; Additional reporting by Camillus Eboh; Writing by Libby George; Editing by Peter Graff)