The Pentagon is running an ongoing mission in which US forces work with African Union and Somali national security forces to fight the Shabaab movement.

"At the end of the day, these strikes are not going to defeat al-Shabaab," the US military's Africa Command head General Thomas Waldhauser told the Senate Armed Services Committee.

"The bottom line is the Somalian national army needs to grow, it needs to step up and it needs to take responsibility for their own security," he added.

Since early 2017, the US has increased the number of strikes it conducts, with 35 that year, 47 in 2018 and about 12 so far this year, Waldhauser said.

He added that US strikes were providing the space for local forces to grow.

African Union troops pushed Shabaab fighters out of Somalia's capital in 2011, and subsequently from other towns and cities.

But the Islamists still hold sway in large parts of the countryside.

They launch regular gun and bomb attacks on government, military and civilian targets in Mogadishu as well as ambushes on military convoys and outposts.

In one strike last month, Africa Command said it killed 52 Somali militants.

In December, the US military said it had killed 62 Shabaab militants during six air strikes in the Horn of Africa nation.