Johannesburg: South African police say a shipment of guns bound for Somalia was moved from Durban's port into a house outside the city under mysterious circumstances.

Police spokesman Vincent Mdunge said on Tuesday that police had been tracking the shipment and that four people were arrested on weapons charges last week after the guns were traced to the house.

Thirteen guns were found and Mdunge says others may still be hidden.

Mdunge says police are trying to determine why the weapons were moved from port.

He says investigators believe the weapons were being sent to Somalia for an anti-piracy campaign, but did not know whether the Somali government or a private company was the importer.

He says it was unclear whether the shippers had the proper permits to send the weapons to Somalia.