MUMBAI, INDIA -- A growing number of current and recovered COVID-19 patients in India are contracting a deadly and rare fungal infection, doctors told AFP Monday as the vast nation battles to contain a massive outbreak of virus cases.

The world's second most populous nation on Monday reported nearly 370,000 coronavirus infections to take the overall number of cases to just under 22.7 million, and more than 3,700 new deaths.

Experts warn that the actual number of cases and fatalities could be much higher.

Mucormycosis, dubbed "black fungus" by doctors in India, is usually most aggressive in patients whose immune systems are weakened by other infections.

Medical experts said they had seen a rise in cases in India in recent weeks, while the health ministry on Sunday released an advisory on how to treat the infection.

"The cases of mucormycosis infection in COVID-19 patients' post-recovery is nearly four to five times than those reported before the pandemic," Ahmedabad-based infectious diseases specialist Atul Patel, a member of the state's COVID-19 taskforce, told AFP.

In the western state of Maharashtra, home to India's financial hub Mumbai, up to 300 cases have been detected, said Khusrav Bajan, a consultant at Mumbai's P.D. Hinduja National Hospital and a member of the state's COVID-19 taskforce.

Some 300 cases have been reported so far in four cities in Gujarat, including its largest Ahmedabad, according to data from state-run hospitals.

The western state ordered government hospitals to set up separate treatment wards for patients infected with "black fungus" amid the rise in cases.

"Mucormycosis -- if uncared for -- may turn fatal," the Indian Council for Medical Research (ICMR), the scientific agency leading the government's response, said in a treatment chart released on Twitter.

COVID-19 sufferers more susceptible to contracting the fungal infection include those with uncontrolled diabetes, those who used steroids during their virus treatment, and those who had prolonged stays in hospital ICUs, the ICMR added.

Treatment involves surgically removing all dead and infected tissue and administering a course of anti-fungal therapy.

But Yogesh Dabholkar, an ear, nose and throat specialist at Mumbai's DY Patil Hospital, told AFP that the drugs used to treat those infected with the fungus were expensive.

One of the treatment drugs was also running short in government hospitals due to the sudden spike, he added.

"The mortality rate is very high... Even the few that recover, only recover with extensive and aggressive surgery," Bajan said.

"This is a fast-moving infection. It can grow within two weeks... It's a Catch-22, coming out of a virus and getting into a fungal infection. It's really bad."