Five people died as a fire tore through a building in the world's biggest vaccine manufacturing hub in India Thursday, but the company insisted that the production of drugs to counter the coronavirus pandemic would continue.

Rescue workers discovered five bodies in the under-construction building after the blaze at the Serum Institute of India was brought under control, media reports said, with officials in the western city of Pune confirming the toll.

"Five people have died," city mayor Murlidhar Mohol told reporters.

TV channels showed thick clouds of grey smoke billowing from the sprawling site in Pune, which is responsible for producing millions of doses of the Covishield coronavirus vaccine, developed by AstraZeneca and Oxford University.

"It is not going to affect production of the Covid-19 vaccine," a Serum Institute source told AFP, adding that the blaze was at a new facility being built on the 100-acre (40-hectare) campus.

A fire official earlier told reporters that "three or four people were inside" when the blaze broke out, but all had been rescued.

"No-one is trapped inside," the official said.

"We are deeply saddened and offer our deepest condolences to the family members of the departed," the firm's CEO Adar Poonawalla tweeted, without offering further details.

Both police and the company said the cause of the blaze was not immediately known.

The complex where the fire broke out is a few minutes' drive from the facility where coronavirus vaccines are produced, reports said.

Up to nine buildings are under construction at the complex to enhance Serum's manufacturing capability, NDTV reported.

Serum Institute -- founded in 1966 by Cyrus Poonawalla, Adar's father -- is the world's largest vaccine manufacturer by volume, producing 1.5 billion doses a year even before the coronavirus pandemic hit.

It makes vaccines against polio, diphtheria, tetanus, hepatitis B, measles, mumps and rubella, which are exported to more than 170 countries.

The company has spent nearly a billion dollars in recent years enlarging and improving the giant Pune campus.

In January, Indian regulators approved two vaccines -- Covishield, produced by the Serum Institute, and Covaxin, made by Indian firm Bharat Biotech.

India began one of the world's biggest vaccine rollouts on Saturday, aiming to vaccinate 300 million people by July with both Covishield and Covaxin.

Many other countries are also relying on the Serum Institute to supply them with the vaccine.

India exported its first batch on Wednesday -- to Bhutan and the Maldives -- followed by two million doses to Bangladesh and a million to Nepal on Thursday.

The country plans to offer 20 million doses to its South Asian neighbours, with Latin America, Africa and Central Asia next in line.

Serum Institute also plans to supply 200 million doses to Covax, a World Health Organization-backed effort to procure and distribute inoculations to poor countries.

Brazil last weekend was set to send a plane to collect two million doses from Serum but President Jair Bolsonaro said that "political pressure" in India had postponed the flight.

Adar Poonawalla told the Times of India it would supply Brazil in two weeks.

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