Militants from the Nigeria-based Islamic extremist group Boko Haram crossed the border into Niger, setting homes ablaze and killing at least 40 people, a top provincial official said Thursday. The official, Yacouba Soumana Gaoh, the governor of Niger’s Diffa region, said the attacks took place overnight in the towns of Lamana and Ngoumawa. This week, Chad blamed the group for two suicide bombings in its capital that left at least 33 people dead. Chad’s military spokesman, Col. Azem Bermandoa Agouna, said a retaliatory aerial assault was carried out Wednesday inside Nigeria on Boko Haram positions, a claim that Nigeria denied.