The state-run Indian Council of Medical Research (ICMR) said at the weekend that doctors treating COVID-19 patients, diabetics and those with compromised immune systems should watch for early symptoms including sinus pain or nasal blockage on one side of the face, one-sided headache, swelling or numbness, toothache and loosening of teeth.

The disease, which can lead to blackening or discolouration over the nose, blurred or double vision, chest pain, breathing difficulties and coughing blood, is strongly linked to diabetes. And diabetes can in turn be exacerbated by steroids such as dexamethasone, used to treat severe COVID-19.

"There have been cases reported in several other countries - including the UK, U.S., France, Austria, Brazil and Mexico, but the volume is much bigger in India," said David Denning, a professor at Britain's Manchester University and an expert at the Global Action Fund for Fungal Infections (GAFFI) charity.

"And one of the reasons is lots and lots of diabetes, and lots of poorly controlled diabetes."

India has not published national data on mucormycosis but has said there is no major outbreak. Media reports have pointed to cases in Maharashtra and its capital Mumbai, and Gujarat.

Aparna Mukherjee, a scientist at ICMR, said: "It's not something to panic about, but you have to be aware of when to seek consultation."

But it is a complication that India's overwhelmed hospitals, desperately short of beds as well as the oxygen needed for severely ill COVID-19 patients, could do without.

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