HONG KONG (AFP) - At least 49 passengers on a flight from New Delhi to Hong Kong have tested positive for Covid-19, the authorities said, as the financial hub introduced an emergency ban on all flights from India.

All of the passengers who tested positive flew into Hong Kong on a flight run by Indian operator Vistara on April 4.

The positive tests are significant as Hong Kong has been regularly recording fewer daily cases than the total detected on the flight since it brought a fourth wave under control in January.

The authorities imposed a two-week ban on all flights from India, Pakistan and the Philippines from Monday (April 19), categorising the countries as "extremely high risk" after detecting the N501Y mutant Covid-19 strain for the first time.

India is battling skyrocketing infections, with hospitals running out of beds and the government forced to reimpose economically painful restrictions.

Its capital New Delhi went into lockdown from Monday night as officials scrambled to get surging cases under control.

The identified Vistara aircraft can hold a total of 188 passengers but the Hong Kong authorities did not announce how many it had been carrying.

The positive coronavirus results surfaced during Hong Kong's mandatory three-week quarantine period, one of the strictest entry regimes in the world.

All travellers flying into the territory must also provide a negative test within 72 hours prior to departure.

Several cases have also been detected on flights arriving in Hong Kong from Mumbai, the authorities said.

Densely populated Hong Kong was one of the first places to be hit by the coronavirus, but the tough entry conditions, strict social distancing measures and universal mask wearing have helped to keep infections to just over 11,000, with 209 deaths.

Around 9.8 per cent of the city's 7.5 million population have been vaccinated so far.