More than 40 people have been killed in a large fire at a factory in the Indian capital, Delhi, officials say. The blaze broke out at the six-storey building in the city's congested old quarter in the early hours of Sunday morning. Dozens of workers were sleeping inside the factory, which manufactures bags, say rescue services. There are fears the death toll will rise further. Home Minister Amit Shah called it a "tragic loss of precious lives". Prime Minister Narendra Modi said the fire was horrific and that authorities were "providing all possible assistance at the site of the tragedy". Firefighters had to carry the victims out one-by-one because of the narrow alleyways Delhi's firefighters received the first call about the fire at 05:22 local time on Sunday (23:52 GMT Saturday). The area where the factory is located - Sadar Bazaar - is the city's largest commercial market. It has many narrow alleyways, which made it difficult to reach the blaze. By 09:30 local time, at least 56 people had been moved to hospital, Delhi Fire Service director Atul Garg was quoted as saying by the Hindustan Times newspaper. "All of them had fallen unconscious because of smoke," he said. Scores of injured people are now being treated in the city's hospitals. A cause for the blaze is yet to be determined Another senior fire service official, Sunil Choudhary, said the factory was used to store school bags, and bottles, among other materials. The cause of the fire is being investigated.