STUTTGART, Germany — A recent flurry of American airstrikes against Islamic State fighters in southern Libya has seriously disrupted the terrorist group’s efforts to reorganize and carry out attacks in one of its most important hubs outside the Middle East, military and counterterrorism officials say.

Over 10 days in late September, four strikes killed 43 militants — or about one-third of the group’s estimated 150 fighters in Libya — including some important commanders and recruiters, according to officials at the headquarters of the Pentagon’s Africa Command.

The strikes, which other officials said were carried out by Air Force MQ-9 Reaper drones based in neighboring Niger, came as the Islamic State had increased recruiting and attacks in recent months in the largely ungoverned spaces of southwest Libya. Several new camps of fighters had emerged in that area, prompting the first American strikes against ISIS in Libya this year.

“The most critical, enduring weakness for both ISIS Libya and A.Q.I.M. is recruitment,” Rear Adm. Heidi Berg, the command’s director of intelligence, said in an interview, referring to the Islamic State and Al Qaeda in the Islamic Maghreb, which is active in Libya.