Over 230 cases of highly infectious variant strains of the novel coronavirus have been confirmed in 19 of the 47 prefectures in Japan, government data showed Thursday, with health experts warning of a potential "fourth wave" of infections should they continue to spread across the country.

The government is monitoring three mutant strains of the coronavirus originating from Britain, South Africa and Brazil. Following the first confirmed case of the new variants in Japan on Dec. 25, 234 have been reported as of Thursday, including those detected at airport quarantine.

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)

Saitama Prefecture accounted for the most cases at 38, followed by Hyogo Prefecture at 36 and Niigata at 29. Some regions reported cases that were part of cluster infections. Although yet to be confirmed, the new strains are also suspected to be present in other parts of Japan including Hiroshima Prefecture.

According to the National Institute of Infectious Diseases, 93 percent of cases reported as of Feb. 25 were of people who had no record of overseas travel. Those in their 30s to 50s accounted for the most cases at 47 percent, followed by those in their 20s or younger at 39 percent, and those in their 60s or older at 14 percent.

Of the three variants, the British strain made up the vast majority of cases at 96 percent.

A health ministry advisory panel on Wednesday analyzed that reported COVID-19 cases and cluster infections were on an upward trend despite the state of emergency still in place over the capital and three neighboring prefectures.

Prime Minister Yoshihide Suga has cited the potential surge of the new variants as one of the reasons he is likely to extend the emergency declaration in the Tokyo metropolitan region for about two weeks.

While the state of emergency was ended early for the other seven prefectures it was declared over, Suga is set to make a decision Friday on whether to extend it for the Tokyo metropolitan region.

Some officials involved with preparations for the Tokyo Olympics and Paralympics have also said allowing overseas spectators for this summer's games would be difficult due to how widespread the new variants are in some regions of the world.

In response to the growing number of cases in Japan, the central government will establish a system where local municipalities can conduct PCR tests for the mutant strains. It is aiming for local governments to test around five to 10 percent of the samples provided by patients infected with COVID-19.