JOHANNESBURG (AP) — A U.S. military airstrike has killed 18 al-Shabab extremists after U.S. and local forces on the ground came under attack in southern Somalia, the U.S. Africa Command said Saturday.

No U.S. or Somali forces were killed or injured in the attack, an AFRICOM spokesman, Nate Herring, told The Associated Press. The airstrike was carried out Friday in self-defense after extremists were “observed maneuvering on a combined patrol,” while the U.S. also responded with “indirect fire,” the spokesman said.

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The confrontation occurred about 50 kilometers (31 miles) northwest of the port city of Kismayo, the U.S. Africa Command statement said. Two other al-Shabab extremists were killed by Somali forces “with small arms fire during the engagement,” it said.

The operation was Somali-led, the AFRICOM spokesman said. There was no immediate comment from Somali authorities.

The U.S. has carried out more than 20 airstrikes this year against the al-Qaida-linked al-Shabab, the deadliest Islamic extremist group in sub-Saharan Africa.

U.S. military involvement in Somalia has grown since President Donald Trump early in his term approved expanded operations against al-Shabab. Dozens of drone strikes followed. Late last year the military also carried out its first airstrike against a small presence of fighters linked to the Islamic State in northern Somalia.

Since the expanded operations, two U.S. military personnel have been killed in Somalia.

A service member was killed in May 2017 during an operation about 40 miles (64 kilometers) west of Mogadishu. And in June, one U.S. special operations soldier was killed and four U.S. service members wounded in an “enemy attack” as troops with Somali and Kenyan forces came under mortar and small-arms fire in Jubaland.

The U.S. currently has about 500 military personnel in the Horn of Africa nation.

Al-Shabab, which seeks to establish an Islamic state in Somalia, was pushed out of Mogadishu in recent years but continues to control rural areas in the south and central regions. Its fighters continue to attack the bases of a multinational African Union force that remains largely responsible for security as Somalia’s fragile central government tries to recover from decades of chaos.

In the next few years Somali forces are expected to take over responsibility for the country’s security as the AU force withdraws. Concerns about their readiness remain high, and the U.N. Security Council recently voted to delay the handover’s target date to December 2021.

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