"A policeman died and several others were injured after a bomb blast. We are investigating the cause of the blast," Hassan Nur, a policeman, told Reuters by phone.

Al Shabaab said the toll was higher. "We planted a bomb inside a police station in Kismayu. We killed four policemen and wounded 27 others," its military operations spokesman Abdiasis Abu Musab said.

Al Shabaab lost control of Kismayu in 2012, depriving it of a key source of funds. It had ruled most of south-central Somalia until 2011, when it was driven out of the capital Mogadishu by African Union troops.

Despite the loss of territory, al Shabaab still carries out major gun and bomb attacks, and often claims casualty numbers that conflict with those given by government officials.