More than 2o people suffered minor injuries on Sunday after an explosion and fire on board a ship at a naval base in San Diego, military officials said. The blaze was reported shortly before 9am on USS Bonhomme Richard, said Mike Raney, a spokesman for the navy’s Pacific fleet. Four civilians and 17 sailors were taken to hospital with non-life threatening injuries, Raney said. The cause of the fire is under investigation. Raney did not say where on the 840ft vessel the explosion and fire occurred. San Diego is the Bonhomme Richard’s home port and it was undergoing routine maintenance at the time of the fire. The flames sent up a huge plume of dark smoke visible around San Diego. Two other docked ships, USS Fitzgerald and USS Russell, were moved to berths away from the fire, Raney said. Raney estimated about 160 sailors and officers were on board, far fewer than if the ship was on active duty. The ship has the capacity to deploy and land helicopters, smaller boats and amphibious vehicles.