Japan's total number of confirmed COVID-19 cases topped 500,000 on Friday, as the country braces for a possible fourth wave of infections.

The spread of infections has maintained a rapid clip, taking just two months for the number to increase by 100,000 from 400,000 in early February, while vaccines remain widely unavailable in the country, which is now grappling with the emergence of more contagious variants.

People wearing face masks to protect against the novel coronavirus walk in Tokyo on April 9, 2021. The Japanese government designated Tokyo and Kyoto and Okinawa prefectures the same day as requiring stronger measures to fight COVID-19 amid a resurgence in infections, less than three weeks after fully lifting a state of emergency. (Kyodo) ==Kyodo

On Friday, Japan's daily nationwide tally was more than 3,000 cases for the third straight day, with the government adding Tokyo and Kyoto and Okinawa prefectures to the list of being designated as on the brink of a state of emergency, allowing local authorities to take stronger anti-virus measures.

The resurgence came on the heels of a gradual lifting of a state of emergency for the Tokyo metropolitan region and other parts of the country.

In early January, when Japan's single-day number reached nearly 8,000, the second state of emergency was declared for the Tokyo region, ordering restaurants and bars to cut their business hours, and it was later expanded to a total of 11 prefectures.

Tokyo, which was last to see the end of the emergency last month, on Friday reported more than 500 new coronavirus cases for the third day in a row.

The fourth wave is thought to be partly driven by new variants of the virus, spreading particularly in the western prefectures of Hyogo and Osaka, which came under a quasi-state of emergency on Monday.

As of early April, among COVID-19 patients, around 70 percent of them in the two prefectures are estimated to have been infected with new variants, compared with roughly 10 percent in Tokyo and nearby Kanagawa and Saitama prefectures, according to the National Institute of Infectious Diseases.

Vaccination remains largely unavailable in Japan, with the government beginning the rollout in February for health workers and planning to expand the program to the rest of the population, starting off with people aged 65 or older from Monday.