Indian megacities Delhi, Mumbai and Kolkata are experiencing a surge in Covid-19 cases, although without a corresponding rise in hospitalisations, but fears are growing about a spread to rural areas in coming days.

India reported 90,928 new daily Covid-19 cases on Thursday, up nearly four-fold since the start of the year, mostly from cities where health officials say the Omicron variant has overtaken Delta. The bulk of those infected have shown no or only mild symptoms and have recovered quickly at home, officials said.

The federal health ministry on Wednesday identified Delhi, Mumbai, Kolkata, Chennai and Bengaluru as some of the main regions of concern, although state officials worry the disease will soon spread to the countryside where health facilities are weaker.

Kolkata, a city of about 15 million, accounted for half of the new cases in the eastern state of West Bengal until a few days ago, but cases are now rising in neighbouring districts. The state has reported one of the highest rates of infections in India.

"We are watching the situation in the districts and rural belts where the numbers are also growing," said Ajay Chakraborty, director of the West Bengal health services who has isolated himself at home after contracting the virus.

Many Covid beds in Kolkata were still empty, Chakraborty said. In the government-run Infectious Diseases and Beliaghata General Hospital, only 75 admissions were recorded on Tuesday despite more than 9,000 new cases, he added.

In the west, Mumbai recorded a new daily infection peak of 15,166 on Wednesday, well up on its previous high of just over 11,000 hit last year. Nearly 90 percent of new patients had shown no symptoms and only 8 percent were hospitalised, city officials said in a daily health bulletin.

Covid-19 cases nearly doubled in a day in Delhi to 10,665 on Wednesday, but the state said only 7 percent of its Covid beds were occupied.

Federal health officials, however, have warned even a large number of mild cases could put pressure on the health system.

India has confirmed at least 2,135 Omicron cases and one death linked to the variant, in an elderly man who was suffering from diabetes.

Daily Covid-19 deaths rose by 325 on Thursday, taking the total to 482,876. Total infections are at 35.11 million, only behind the U.S. tally.

As cases rise, the western state of Gujarat on Thursday indefinitely postponed a Jan. 10-12 biennial investment summit that was to be inaugurated by Prime Minister Narendra Modi and previously attended by the country's top billionaires. Modi's home state reported 3,350 infections on Wednesday.

Many Indian cities have already imposed night curfews and weekend lockdowns, as well as closed schools. Political rallies, however, have continued in several states where elections are due in the next few weeks and months.

Health authorities will discuss the matter with election commission officials on Thursday amid rising concerns about such rallies that led to a devastating second wave in the country in April and May.