PUNE, India - Five people were killed in India on Thursday (Jan. 21) when a fire broke out in a facility belonging to the world's biggest vaccine-maker. 

According to CNN, the deadly blaze occurred at the Serum Institute of India (SII) in the western city of Pune. It was brought under control on Thursday, but the cause of the fire remains under investigation, according to Murlidhar Mohol, the city's mayor.

Four people were rescued from the six-floor building but five others died, Mohol said. They are believed to have been construction workers as the building was still under construction at the time of the fire.

Videos and images showed black smoke billowing out of the building at the company's complex. Fifteen units of the municipal corporation and fire department worked to douse the fire, CNN reported.

The news outlet added that a preliminary investigation suggests that "during the building's construction, some welding work could have led to the fire," according to Mayor Mohol.

Pune's fire brigade chief Prashant Ranpise said it took a crew of about 50 firefighters to extinguish the blaze.

"We have learnt that there has unfortunately been some loss of life at the incident. We are deeply saddened and offer our deepest condolences to the family members of the departed," SII CEO Adar Poonawalla tweeted Thursday.

According to CNN, SII is the world's biggest vaccine maker and is in partnership with Oxford University and AstraZeneca to produce the Covishield vaccine.

In December, the company said it was producing 50 to 60 million doses of Covishield per month, with production to be scaled up to 100 million doses in January or February, CNN reports.

Cyrus S. Poonawalla, SII's chairman and managing director, said in a statement that the fire broke out at a facility that was under constriction in the Special Economic Zone at Manjri. He said it was an "extremely sorrowful day" and the company would offer INR 2.5 million ($34,000) to each of the victims' families.

Indian Prime Minister Narendra Modi also tweeted his condolences Thursday, saying, "Anguished by the loss of lives due to an unfortunate fire ... In this sad hour, my thoughts are with the families of those who lost their lives. I pray that those injured recover at the earliest."