WASHINGTON -- The US military has conducted an airstrike targeting a militant group leader in Somalia, the Pentagon said Monday.

According to Pentagon press secretary Rear Admiral John Kirby, "the strike took place in the vicinity of Saakow, Somalia", targeting a senior al-Shabaab leader, but he did not mention the name.

The US Defense Ministry said in a statement that the military is still assessing the results of the operation, adding that at this time there is no reason to believe there were any "civilian or bystander casualties".

Al-Shabaab, which means "The Youth", is believed to be an al-Qaeda-linked extremist group and designated as a terrorist organization by the United States.

Al-Shabaab is thought to be behind a bunch of notably terror attacks in recent years, with the deadliest ones including multiple-bombing in Uganda's capital in 2010 and a raid on a Nairobi mall in 2013.

The group last week claimed responsibility for an attack on the main African Union (AU) base in Mogadishu on Christmas Day. The attack killed at least three soldiers and one civilian.

A spokesman for the group said the attack was to avenge the death of Al-Shabaab leader Ahmed Godane, who was killed in a US airstrike in September.