Just over a week before the opening ceremony of the postponed Games, seven staff at the hotel in Hamamatsu city, southwest of Tokyo, had tested positive, a city official said.

But a 31-strong Brazilian Olympic delegation, which includes judo athletes, are in a "bubble" in the hotel and separated from other guests and have not been infected.

Medical experts are worried that Olympic "bubbles", imposed by Tokyo 2020 Olympic officials in an effort to keep out COVID-19, might not be completely tight as the movement of staff servicing the Games can create opportunities for infection.

The Olympics have lost much public support in Japan because of fears they will trigger a surge of infections even though no spectators will be allowed into sports venues.

International Olympic Committee (IOC) President Thomas Bach praised the organisers and the Japanese people for staging the Games in the midst of the pandemic.

"These will be historic Olympic Games ... for the way how the Japanese people overcame so many challenges in the last couple of years, the great east Japan earthquake and now the coronavirus pandemic," Bach told reporters after meeting Prime Minister Yoshihide Suga.

When Japan was awarded the Games in 2013, they were expected to be a celebration of recovery from a deadly earthquake, tsunami and nuclear accident in 2011.

When they were postponed last year Japanese leaders had hoped they would be a celebration of the world's victory over the coronavirus but those celebrations are on hold as many countries struggle with new surges of infections.

Host city Tokyo reported 1,149 COVID-19 cases on Wednesday, its highest daily tally in nearly six months.

Bach reiterated that strict coronavirus measures were "in place and they are enforced and they are working", referring to extensive testing. A coronavirus state of emergency is also in force in Tokyo until after the Games end on August 8.

The coronavirus cluster at the Brazilians' hotel was found during routine screening required before staff started work, said city official Yoshinobu Sawada.

Many Olympic delegations are already in Japan and several athletes have tested positive upon arrival.

Members of the South African rugby team are in isolation after arriving, as they are believed to be close contacts with a case on their flight, said Kagoshima city, which is hosting the team.

The 21 members of the South African squad were due to stay in the city from Wednesday, but that plan has been halted until further advice from health authorities, said city official Tsuyoshi Kajihara.

With spectators barred from all Olympic events in Tokyo and surrounding regions officials are asking people to watch the Games on television and keep their movements to a minimum.

Australian Associated Press