Niamey - Boko Haram killed at least two soldiers and wounded seven others in a "surprise attack" on a military base in southeast Niger that began overnight, a local official said Saturday.Members of the Islamic State group affiliate arrived in "great numbers" on foot after crossing the Komadougou Yobe river at the Nigerian border, the official told AFP on condition of anonymity.The attack targeted a base in Geskerou in the Diffa region, with the assailants also torching three army vehicles. A security official confirmed the two soldiers were members of Niger's national guard based in Diffa, a Boko Haram stronghold close to the border with Nigeria.In response, Niger launched air strikes, killing "many terrorists" as they fled through the marshes of Lake Chad, which straddles the central African countries of Niger, Chad, and Nigeria, the security source said. Another three Niger soldiers were killed on New Year's Eve when Boko Haram attacked their position at Baroua, also in the Diffa region, the military said. Last month, the Niger government claimed that 50 Boko Haram fighters had surrendered, 31 of whom turned themselves in at Diffa. Such claims are becoming increasingly common and it was not possible for AFP to verify the information. The Nigeria-based Islamist group has waged a seven-year uprising that has claimed more than 20,000 lives, with the insurgency spilling over the West African nation's borders into neighbouring states.The violence has left around 2.6 million people homeless, sparking a dire humanitarian crisis, with the United Nations warning the affected region faces the "largest crisis in Africa". Diffa is one of the worst-hit regions, with around 300,000 people supported by a local population which is already chronically poor.The UN Security Council announced last week that it is planning a visit in the coming weeks to Nigeria, Cameroon, and Chad.