HYDERABAD, India (AP) — Fire raged through a pharmaceutical factory after an explosion in southern India, killing at least six workers on night shift and burning 13 others, police said Thursday.

The fire broke out due to a chemical reactor leaking monomethyl nitric acid — used for manufacturing several types of polymers — which caused an explosion on Wednesday night, Police Officer Rahul Dev Sharma said.

Five workers died on the spot and another succumbed to burns in a hospital. The 13 hospitalized workers were in critical condition as they suffered 80% burns, Sharma said.

The cause of the leak is being investigated, he said. The chemical reactor was completely damaged, but firefighters saved other parts of the factory from being destroyed by the fire, Sharma said.

India's Prime Minister Narendra Modi said he was pained by the loss of lives due to a mishap at the Porus Labs chemical factory, located in Akkireddygudem, a village in Andhra Pradesh state.

"My condolences to the bereaved families. May the injured recover quickly," he tweeted.

Porus Labs is privately owned and specializes in the development and production of pharmaceuticals and specialty chemicals that are used by drugmakers, its website said.

India is a leading global pharmaceutical producer, including making COVID-19 vaccines.

Fires are common in India, where building laws and safety norms are often flouted by builders and residents. Some don’t even install firefighting equipment.

In 2019, a fire caused by an electrical short circuit in a New Delhi factory producing handbags and other items killed 43 people. In a second major fire that year, a blaze in a six-story building’s illegal rooftop kitchen killed 17 people, also in New Delhi.