A coronavirus cluster at a Japanese hotel where dozens of Brazilian Olympic team members are staying has raised new concerns about infections at what the world's top Olympics official promise will be "historic" Games, reports Reuters.

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.