The U.S. military says it killed at least nine al-Shabab militants in an airstrike in southern Somalia that was conducted after an insurgent attack on government forces.

A statement by the U.S. Africa Command said at least one al-Shabab militant was wounded in the airstrike, which was carried out in coordination with the Somali government on Monday, about 40 kilometers northeast of the southern city of Kismayo.

"We currently assess no civilians were injured or killed in this air strike", the statement said.

The U.S. has conducted more than 20 such airstrikes in Somalia this year.

Al-Shabab, which is allied with al-Qaida, has been fighting since 2006 to overthrow the Somali government and turn the country into a strict Islamic state.