Tokyo Olympics organisers warned Friday they were prepared to hold the Games behind closed doors as virus cases rise, leaving ticket-holders in limbo just three weeks before the opening ceremony.

It came the day after Prime Minister Yoshihide Suga also said that empty venues remain a possibility at the pandemic-postponed Games.

Organisers decided last month to set a limit of 10,000 domestic fans, or half of each venue's capacity, for Olympic events.

But a rebound in Covid-19 cases in Tokyo has sparked fears that spectators could spread infections, with the government expected to extend anti-virus measures covering the capital.

"The infection situation changes on a day-to-day basis, and it's still unclear what the situation will look like," Games president Seiko Hashimoto told reporters.

"But from Tokyo 2020's perspective, having no spectators is an option so that we can be prepared for whatever the situation may be."

Local media reports said Olympic chiefs would meet next Thursday to discuss the matter.

Acknowledging the "inconvenience and concern" among fans who have already bought tickets for events, Hashimoto pledged to make a decision "quickly".

Japan's virus outbreak has been comparatively mild, with around 14,800 deaths, but experts warn another wave of cases could stretch medical services as the Games begin.

Just over 11 percent of the population is fully vaccinated so far.

Cases have been rising since a state of emergency was lifted last month and replaced with softer restrictions, which allow up to 5,000 spectators at sporting events.

Those measures are set to expire on July 11, but the government is expected to extend them as early as next week -- meaning they could still be in place when the Games open on July 23.

Hashimoto said any decision would be taken in line with government policy.

"It's not that we are determined to have spectators regardless of the situation," Hashimoto said.

"We will follow the government standards. Unless it's safe and secure, we cannot allow spectators."

But she could not guarantee that a ticket lottery for oversubscribed events, due to take place on Tuesday, would go ahead as scheduled.

Organisers need to reclaim around 900,000 of the 3.6 million tickets already spoken for, with reductions required in eight sports and the opening and closing ceremonies.

No more tickets will be sold and spectators from overseas have already been barred from attending.

Japanese media reported that several options will be on the table when local organisers meet officials from the International Olympic Committee, International Paralympic Committee, the national government and Tokyo government.

The Yomiuri Shimbun daily said Friday that spectators could be barred from events held in the evening or in large venues, citing unnamed sources.