Tokyo is under a fourth state of emergency, which began on Monday and requires restaurants and bars to close early and not serve alcohol through the Olympics, which start July 23.

Thursday's tally is the highest since 1485 were recorded on January 21, when Japan was under an earlier state of emergency, and is also a jump from Wednesday's 1149.

Tokyo Governor Yuriko Koike noted the largest increase in serious cases and hospitalisations was among people in their 50s and younger who are largely unvaccinated.

She expressed concern about the impact on the medical system as infections are propelled by the more contagious delta strain of the virus.

"We need to stay on alert," Koike said, urging people to minimise outings and stick to basic anti-infection measures "to overcome this very difficult situation".

New daily cases have been steadily climbing since mid-June and experts say they could hit several thousand during the games.

Japan's slow vaccination rollout has improved dramatically since May as the government desperately pushes to improve the inoculation rate before the Olympics, but is slowing again due to shortages of imported vaccines.

The latest government data show just 19.7 per cent of the population has been fully vaccinated.

Due to the state of emergency in Tokyo and a fear of infections accelerating during the games, organisers last week decided to bar fans for most events, except for limited numbers at outlying locations.

Overall, Japan has had about 828,000 confirmed cases and 15,000 deaths.

Australian Associated Press