(CNN) At least 43 people have died in a major blaze that broke out in a factory in India's capital New Delhi, police and fire department officials told CNN.

The fire swept through the five-story building in the early hours of Sunday, as laborers and factory workers were asleep inside. Some died due to asphyxiation, officials said.

"The problem was the smoke -- all the windows and doors were shut and there was dense smoke inside," said Sunil Choudhary, deputy chief fire officer.

"No one could get out. There was an iron door and it was locked and people were brought out only after we broke the door open. It had become a toxic chamber."

Police personnel try to clear a road as onlookers gather near the factory fire.

An eyewitness said some workers had tried to escape from a terrace, but it was locked.

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