Seven female workers were killed yesterday morning when locked doors and windows prevented them from escaping a fire that swept through an unlicensed factory in Shenzhen's Gongming township. A further four workers were sent to hospital suffering from smoke inhalation. About 11 women were working in the underwear factory on the second floor of a seven-storey building when the fire broke out at 8am. The blaze is believed to have been started by the burning of surplus material and rapidly spread through the plant as people were working. Witnesses said workers were prevented from escaping as the factory windows had been blocked with metal screens designed to prevent break-ins. When, firefighters arrived about 20 minutes after the fire broke out, they found the main entrance to the factory locked. They were unable to enter the building until the blaze had been brought under control half an hour later. Firefighters then climbed to the second floor and broke windows to rescue the workers. A neighbour said several female workers were trapped inside and could be heard crying for help. 'The fire was so fierce,' he said. 'We could do nothing but watch them die. It was so heart-breaking hearing them call out.' A local official said about 16 people were employed at the factory, which opened three months ago. Eleven people were working overtime at the plant when the tragedy occurred. The owner of the factory has been detained. The third to seventh floors of the building were being rented by the Skyworth Group, a large hi-tech company, as a dormitory for its workers. No Skyworth workers were hurt as most of them had left the dormitory for work before the fire broke out. It is not the first time that locked exits and a lack of fire escapes have resulted in tragedy at a private factory in Guangdong. Last year, a fire killed 13 women in Shantou as the windows on the top three stories of their factory had been covered with iron bars and the only emergency exit had been locked. In 1993, 87 workers died in a fire that broke out in a Shenzhen factory. Officials have warned it would take more than a decade for China to come out of the shadow of high workplace fatalities during its rapid industrialisation.