Several recent military airstrikes against Islamic State fighters in southern Libya have seriously disrupted the terrorist group’s efforts to reorganize and carry out strikes there.

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.

The setbacks for ISIS in Libya come as the global terrorist group is scrambling to regain momentum after the death of its leader Abu Bakr al-Baghdadi in an American commando raid in northwestern Syria on Oct. 26 and the announcement soon after of Mr. al-Baghdadi’s successor, Abu Ibrahim al-Hashemi al-Qurayshi.

Even after Mr. al-Baghdadi’s death — a major blow to the terrorist group — top American counterterrorism officials acknowledged that the Islamic State had begun to rebound, particularly in Iraq, after an American-backed campaign there recaptured territory that made up the group’s religious state, or caliphate. The group’s far-flung affiliates — in places like West Africa, Sinai and the Philippines — have also demonstrated resiliency in their operations, officials said.

“The overall threat from radical Islamist terrorists has not abated and, in some regions, is growing,” Russell Travers, the acting head of the National Counterterrorism Center, told Congress this month. “Prominent groups including ISIS and Al Qaeda are expanding into new areas and reinforcing their networks’ cohesion, bolstering the overall movement’s reach, resiliency and threat to U.S. interests.”

He noted that “enduring conflicts in several countries including Egypt, Mali, Nigeria, Libya, Syria and Yemen continue to serve as incubators for terrorist presence.”

The heightened American counterterrorism operations in southern Libya come against the backdrop of a four-year civil war being fought in the north, a fight in which Russia is now pushing far more directly to shape the outcome.

At least in southern Libya, the recent drone strikes have put ISIS on the defensive. “The group has not conducted attacks, even in its usual area of operations in the center and southwest, since the strikes,” said Emily Estelle, the research manager of the American Enterprise Institute’s critical threats project in Washington.

In news releases, the Africa Command cited airstrikes on Sept. 19, 24, 26 and 29 that it said killed a total of 43 militants. The command said, for instance, that the strike on Sept. 19 killed eight ISIS fighters in a compound in Murzuq, Libya, nearly 600 miles south of Tripoli, Libya’s capital. Five days later, the military said it killed 11 more fighters in an airstrike in the same area.

Social media reports in Libya said that among the militants targeted in the strikes was Malik Khazmi, a major ISIS facilitator and recruiter from Bani Walid. Africa Command officials declined to confirm whether Mr. Khazmi had been among the top ISIS fighters killed.

Independent security analysts said that Mr. Khazmi had been an important ISIS recruiter and architect of its clandestine fighter networks since 2014, surfacing in pivotal combat areas like Darnah, Tripoli and Surt, before fleeing into the southern desert.

Taken together, the four missile attacks were the first American airstrikes this year in Libya against Islamic State or Qaeda fighters, after the military conducted six aerial attacks last year, most recently in November 2018.

“We need to make sure we look at ungoverned spaces where versions of ISIS can pop up,” Gen. Jeffrey L. Harrigian, the commander of United States air forces in Europe and Africa, said in an interview at his headquarters at Ramstein Air Base, Germany. “We’re doing good work down there, pressuring them.”

Regional experts have cautioned that in the remote and politically fractured landscape of southern Libya, the line between who is a militant or terrorist or militiaman is frequently blurred, and there is the potential for these strikes to err and inflame ethnic and tribal tensions.

“The most important thing to realize is that the effect of the strikes is only temporary,” Ms. Estelle said.

“The conditions that support ISIS’s return are still in place due to the war and the local fallout, including social tensions and popular grievances in the southwest where ISIS already has a foothold,” she added. “ISIS in Libya may be hindered for a few months, but it will rebuild repeatedly and emerge in new forms as long as Libya is in crisis.”

American counterterrorism operations in southern Libya entered a new phase when a military drone strike in March 2018 killed a top recruiter and logistics specialist for Al Qaeda’s branch in northwest Africa. That attack killed two militants, including one identified by American officials as Musa Abu Dawud, a high-ranking official in Al Qaeda in the Islamic Maghreb.

Until that strike, the Pentagon had focused its counterterrorism strikes in Libya almost exclusively on Islamic State fighters and operatives farther north. Over several months in 2016, the military conducted nearly 500 airstrikes in the coastal city of Surt to destroy the Islamic State’s stronghold there.

Drones remain the weapon of choice to attack the militants in Libya. The military launches its MQ-9 Reaper drones from bases in Sicily and in Niamey, Niger’s capital. It is also building a new $110 million drone base in Agadez, Niger, which is much closer to militant hotbeds in southern Libya than the base in Niamey. The Africa Command announced earlier this month that reconnaissance flights had started from the base. But military officials said privately they were still working out the logistics of arming the aircraft with missiles.

The C.I.A. is also broadening its drone operations in the region, moving aircraft to northeastern Niger to hunt militants in southern Libya. Nigerien and American officials told The New York Times last summer that the C.I.A. had been flying drones on surveillance missions for several months from a corner of a small commercial airport in Dirkou.