TOKYO (BLOOMBERG, AFP) - Japanese Prime Minister Yoshihide Suga said the government must be on high alert as virus cases begin to rise in the capital, about three weeks before Tokyo hosts the Olympics.

"While there is a downward trend across the country as a whole, there is a slight upward trend in the capital region," Mr Suga told reporters on Monday (June 28). "We must be on a high state of alert in dealing with the virus."

He added that he would be nimble in adjusting policies to deal with the situation.

Case numbers in Tokyo have been creeping up over the past week, since Mr Suga lifted a state of emergency imposed to rein in infections. Any sharp increase could mean the emergency is reintroduced, further restricting residents' activities even while the games are taking place.

The seven-day moving average of new virus infections recorded in Tokyo rose to 477 on Sunday, compared with 388 the previous week.

Mr Suga's comments came as the capital's government announced that Tokyo Governor Yuriko Koike would take a few more days away from public duties on the advice of a doctor, extending an already week-long absence.

Ms Koike had been expected to return to work on Monday, after taking time off to recuperate from fatigue. She was taken to a hospital last Tuesday, Kyodo News reported, without saying whether she had received a specific diagnosis.

Ms Koike, 68, has led Tokyo's response throughout the coronavirus crisis, often appearing to adopt a more cautious stance than Mr Suga's government.

She has retained relatively strong public backing, with 57 per cent of respondents to a poll of Tokyo residents published by the Asahi newspaper on Monday saying they supported her.

Ms Koike said on June 7 she had received her first dose of a coronavirus vaccine.

The political veteran has stepped out of the spotlight at a critical time, just before local elections in Tokyo on July 4. Ms Koike's term as governor has three more years to run, but she must work closely with the victorious parties in the assembly, which may include Mr Suga's Liberal Democratic Party.

Controversy continues to rage over the Olympics, which are set to open July 23. About 58 per cent of respondents to a Mainichi newspaper survey published on Monday said they opposed the games.

Japan's Imperial Household Agency even weighed in, saying last week that Emperor Naruhito may have concerns that the Olympics could cause coronavirus infections to rise. Mr Suga subsequently dismissed the claims, saying the comments were the opinions of an imperial household representative.

About 55 per cent of respondents to the Asahi survey said they approved of Ms Koike's handling of the virus, compared with 35 per cent who said they did not. Asked about her handling of the Olympics, voters were evenly divided, with 42 per cent saying they approved and the same percentage disapproving.

Ms Koike, a former TV news anchor as well as defence and environment minister, left her seat as a Liberal Democratic Party member of parliament in her successful bid to become the first female governor of Tokyo in 2016.

Her party challenged then-prime minister Shinzo Abe's Liberal Democratic Party unsuccessfully in parliamentary elections the following year.