A drone strike killed a leader of the militant group al-Shabab in southern Somalia on Saturday, according to U.S. officials, in the latest sign of how U.S. military operations in the country are gradually escalating.

Yusef Dheeq, al-Shabab’s head of external operations, died in the attack near the town of Dinsoor, U.S. defense officials said Tuesday, speaking on the condition of anonymity to discuss details of the strike. Dheeq was riding in a vehicle with fellow al-Shabab members, but it was unclear how many others may have been killed, the officials said.

Rear Adm. John Kirby, a Pentagon spokesman, confirmed that Dheeq was targeted in the strike but said the military was still assessing the results and not ready to definitively announce his death.

“If he no longer breathes, then this is another significant blow to al-Shabab and their ability to conduct, plan, prepare for and strike against targets inside and outside Somalia,” Kirby told reporters. “So we would deem this to be a successful — a very significant blow against their capabilities.”

The attack is at least the third U.S. military drone strike in Somalia since September, a small but steady increase after years of sporadic counterterrorism operations there.

A September report from the U.N. Monitoring Group on Somalia identified Dheeq as a “high-profile” commander and the chief of external operations for al-Shabab. The report said Dheeq and other leaders of the group “enjoy operational freedom to plan and direct attacks” from their base in the port city of Barawe.

Since then, the U.S. military has slowly picked up the pace of its attacks. On Sept. 2, the group’s overall leader, Ahmed Abdi Godane, was killed when U.S. drones and other military aircraft launched an attack on an al­Shabab compound near Barawe.

Analysts described Godane as a particularly ruthless Islamist militant who routinely purged al­Shabab’s ranks to enforce loyalty. He had carried a $7 million bounty on his head in the form of a reward posted by the State Department for information leading to his arrest or capture.

About four months later, Tahlil Abdishakur, whom U.S. officials identified as al-Shabab’s intelligence chief, was killed in a strike while he was riding in a vehicle near the southern town of Saakow. The U.S. government had posted a $3 million reward for information on his whereabouts.

Al-Shabab means “the youth” in Arabic and is an Islamist militant movement affiliated with al­Qaeda. Born in Somalia, a chronically unstable country on the Horn of Africa, it has transformed itself from a domestic insurgency into a regional terrorist group that has carried out attacks in Kenya and Uganda. The network has cooperated with al-Qaeda’s franchise in Yemen.

The U.S. military often conducts drone surveillance flights over Somalia, but airstrikes and ground raids had been relatively uncommon prior to last fall. The Pentagon has a large drone base at Camp Lemonnier in Djibouti, which borders Somalia on the Horn of Africa. The U.S. military also flies surveillance drones over Somalia from a base in Ethi­o­pia.

The Pentagon quietly deployed a small team of advisers to Mogadishu, the capital of Somalia, in October 2013 to coordinate operations with African troops fighting to wrest control of the country from al-Shabab.

The deployment marked the first time regular U.S. troops had been stationed in the war-ravaged country since 1993, when two helicopters were shot down and 18 Americans were killed in the “Black Hawk Down” disaster .