The three units, dubbed oxygen factories, are each the size of a shipping container and can produce 500 litres of oxygen per minute, the British government said.

"We stand with our Indian friends in their fight against Covid-19," Foreign Secretary Dominic Raab said in a statement.

"International collaboration is more essential than ever, and this additional UK support package will help meet India's current needs, particularly for more oxygen."

The new shipment follows a first batch of oxygen concentrators and ventilators from Britain which arrived in India on Tuesday, with more of those supplies coming on Friday.

In total, Britain is sending 495 oxygen concentrators and 200 ventilators to India this week. There was no immediate response from the Foreign Office to an AFP query about when the container-sized factories would arrive.

India's coronavirus death toll shot past 200,000 on Wednesday as a relentless wave of new cases swamped hospitals and sent desperate families out into the streets of New Delhi in search of oxygen supplies and medicine.

The United States says it is sending up to 60 million doses of the AstraZeneca Covid-19 vaccine abroad, with India expected to be among the recipient nations.

Critics have accused Washington of "hoarding" the British-developed vaccine, which is not authorised in the United States and will likely not be required to vaccinate Americans.

Britain itself does not have any surplus vaccines to offer India "at the moment", Health Secretary Matt Hancock told a news conference.

But he said research by experts at Oxford University underpinned the AstraZeneca jab, and "that is the biggest contribution that we can make which effectively comes from British science", along with the oxygen supplies.