MOGADISHU (Reuters) - A loud blast was heard in the former al Shabaab stronghold of Kismayu in Somalia on Tuesday, residents said, and the al Qaeda-linked militants said they had detonated a bomb at a building housing Somali troops.

There was no immediate word on casualties.

"The bomb was planted inside a district administration office building," al Shabaab spokesman Sheikh Abdiasis Abu Musab said, warning of more attacks.

(Reporting by Abdi Sheikh; Writing by James Macharia)