MAIDUGURI, Nigeria (AP) — Thousands of members of Nigeria's home-grown Islamic extremist Boko Haram group strike across the border in Cameroon, with coordinated attacks on border towns, a troop convoy and a major barracks. In Niger, the government has declared a "humanitarian crisis" and appealed for international aid to help tens of thousands of Nigerian refugees driven from their homes by the insurgency. A court-martial this week sentenced 54 soldiers to death by firing squad for refusing to fight the extremists. Boko Haram's threat to neighboring countries was highlighted on Wednesday, when some 5,000 insurgents launched simultaneous attacks on border towns in Cameroon, that country's Ministry of Defense said. Cameroonian troops repelled the attacks and killed 116 militants, while losing a sergeant and a lieutenant, it said, adding that Boko Haram must have suffered additional casualties on the Nigerian side caused by Cameroonian artillery fire. Boko Haram leader Abubakar Shekau long has expressed his international ambitions, saying his group is fighting to make "the entire world" an Islamic state.