A Reuters witness saw huge clouds of smoke above the city following a loud blast and gunfire.
Witnesses said the office was destroyed.
"A speeding 4x4 car ran into the district office," witness Hussein Osman told Reuters. "I ran away and now I see from far the office completely destroyed. I see ambulances carrying casualties but I cannot make out how many dead or injured. I could see five people being taken by ambulance."
There was no immediate claim of responsibility for the attack.
Al Qaeda-linked al Shabaab frequently carry out bombings and gun attacks in the capital Mogadishu and other parts of Somalia.
They are fighting to topple Somalia's Western-backed central government and establish their own rule based on their strict interpretation of Islamic law. (Reporting by Abdi Sheikh Writing by Ingrid Melander Editing by George Obulutsa and Matthew Mpoke Bigg)
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