The fire broke out at the plant's rolling section at 7.30 p.m. local time (2.30 p.m. GMT) Tuesday, engulfing an area of about 1,000 square meters (10,800 sq feet).
Six workers were hospitalized with injuries, and at least six were trapped in the facility when the roof collapsed, the spokesman said.
Earlier reports said 14 people were working at the facility at the time the fire broke out, but officials suspect the number could be as high as 16.
The spokesman said the blaze at the plant was put out at 6.45 a.m. Wednesday, adding firefighting efforts were complicated by fallen parts of the roof. About 300 rescuers are searching for more people under the debris.
MMK managers said the lives of the workers being treated in hospital are not in danger.
Emergency services said infringements of operational procedures in a pickling section, which deals with acids, was a possible cause of the blaze, in which the roof collapsed due to the extremely high temperature.
Investigations into the cause of the blaze are under way, the spokesman said
