TOKYO, Sept. 3 - At least 12 people were injured Wednesday after an explosion at a steel plant in central Japan, an official said. Television pictures showed clouds of black smoke billowing from the Nippon Steel and Sumitomo Metal facility in Tokai. A spokesman for the Aichi prefectural police said the fire department had been alerted to a problem at the plant at 12:46 pm (0346 GMT). A Tokai city official said at least 12 people were injured, with public broadcaster NHK reporting that they all worked at the plant. "There was a small explosion, and the cause was a fire at a coke oven," the city official told AFP, adding he did not know details about the severity of the injuries. The explosion came after other fire troubles at the factory earlier this year. No one was hurt in separate incidents in January, June and July at the same plant where 3,000 employees were engaged in producing steel sheets and pipes. Local media said the incidents prompted local officials to ask the steelmaker to draw up a plan to deal with any safety problems.