The U.S. military believes that some 50,000 Islamic State fighters have been killed since the United States started battling the group more than two years ago, a senior U.S. military official said on Thursday, calling it a “conservative estimate.”

The official, who spoke to Pentagon reporters on condition of anonymity, said the figure showed how the United States was effectively combating the group with U.S.-led coalition airpower and limited U.S. troop deployments in support of local forces.