A policeman stands outside the Westgate Mall in Nairobi in September 2014 during a memorial ceremony to mark the one-year anniversary of the attack on the shopping center. (Carl De Souza/AFP/Getty Images)

U.S. military officials said Friday that they had launched a drone strike against a senior leader of the militant group al-Shabab who was suspected of helping to plan a massacre at an upscale Kenyan shopping mall in 2013.

The officials said they believed Thursday’s strike killed Adan Garaar, an al-Shabab commander whom they described as a ringleader of the attack on Westgate Mall in Nairobi. Garaar was riding in a car with two other suspected al-Shabab members in southern Somalia, near Bardera, when the attack occurred, the officials said.

Army Col. Steve Warren, a Pentagon spokesman, confirmed that the U.S. military conducted a counterterrorism operation against an al-Shabab leader but declined to name the target. He said U.S. officials were still awaiting confirmation on whether the individual was killed.

The U.S. drone strike was the fourth in Somalia since September, a small but steady escalation after years of sporadic activity in the country. The increase co­incides with a decision by the Pentagon in late 2013 to deploy a small team of advisers to Mogadishu, the capital, to coordinate operations with African troops fighting to wrest control of Somalia from al-Shabab.

Prior to that, U.S. troops had not been deployed to Somalia on a regular basis since 1993, when two helicopters were shot down and 18 Americans were killed in the “Black Hawk Down” disaster.

The U.S. military regularly conducts drone surveillance over Somalia from Camp Lemonnier, a U.S. base in neighboring Djibouti, as well as from a smaller airport in Ethi­o­pia.

U.S. drone strikes have started to take a toll on al-Shabab’s leadership, though it is unclear to what degree the attacks have weakened the group’s military capabilities.

On Jan. 31, a U.S. drone strike killed Yusef Dheeq, al-Shabab’s head of external operations, near the town of Dinsoor. In late December, the group’s intelligence chief, Tahlil Abdishakur, was killed in a strike while he was riding in a vehicle near the southern town of Saakow.

The biggest blow to al-Shabab came in September, when a drone strike killed the movement’s overall leader, Ahmed Abdi Godane, near the port city of Barawe.

Al-Shabab, whose name means “the youth” in Arabic, is an Islamist movement affiliated with al­Qaeda. Although the group is based in Somalia, it has also carried out attacks in Uganda and Kenya, including the Westgate Mall attack. Sixty-seven people died during a four-day standoff at the mall between four ­al-Shabab gunmen and Kenyan security forces.

Security analysts identified Garaar as a well-known member of the group who has been involved in the planning of a number of recent attacks across East Africa.

“Adan Garaar was one of the key planners behind the assault on the Westgate mall, just as he has been behind a number of other high-profile attacks, both carried out and foiled, including the attempted major bombing in Kampala last year,” said J. Peter Pham, director of the Atlantic Council’s Africa Center. “He may not be as well-known generally as other Shabab leaders but is certainly no stranger to those tracking the terrorist group’s activities elsewhere in East Africa outside Somalia.”

His death, along with a series of targeted killings over the past year, analysts said, helps to weaken the organization as it struggles to maintain its strongholds across Somalia.

“Targeting the leadership is degrading their capacity. They are basically on the run,” said Abdirashid Hashi, executive director of the Mogadishu-based Heritage Institute for Policy Studies. “But they are still doing their operations. I wouldn’t call this the final act. There are different cells, and the leaders who are killed are replaced.”

Sieff reported from Nairobi.