Japan's health ministry said that 505 new cases of highly contagious coronavirus variants were reported in the week to Tuesday, nearly double the amount reported in the previous seven days, amid a nationwide resurgence of infections.

Adding to the 255 cases reported in the previous week, the total number of people in Japan infected with variants of the virus now stands at 1,880, including those quarantined at airports, the ministry said Wednesday.

Supplied electron micrograph shows a variant strain of the novel coronavirus originating from Britain. (Photo courtesy of Japan's National Institute of Infectious Diseases)(Kyodo)

A case reported in Miyagi Prefecture, northeastern Japan, for the first time means variant strains have been confirmed in 43 of Japan's 47 prefectures.

According to the ministry, Hokkaido reported the largest increase in the number of variant cases this past week at 83, bringing its total to 146 cases. It was followed by Nara Prefecture, up 69 to 81, and Tokyo, up 63 to 122.

The western Japan prefectures of Osaka, where the mutated strains are believed to have already been widespread, and Hyogo saw increases of 55 and 14 cases, respectively. However, the number of cases in Kyoto Prefecture decreased by 32 from the previous week due an amendment in figures, the ministry said.

With over 5,000 new cases reported nationwide on Wednesday for the first time since Jan. 22, the central government plans to impose a fresh coronavirus state of emergency in the three prefectures to curb the virus' further spread.

According to a health ministry panel of experts, coronavirus variants account for about 80 percent of infections in Osaka and Hyogo prefectures, as well as a rapidly growing share of cases in Tokyo.

The government is also monitoring the N501Y mutation of the coronavirus, which permits it to be more transmissible than the ancestral strain, and is present in the British, South African and Brazilian variants.

The vast majority of variant cases reported in Japan are of the British strain, which accounts for 1,562 cases of the total confirmed to date.