The public will be urged not to line the route of the Olympic marathon over fears that crowds of fans could spread the coronavirus, Tokyo 2020 organisers said Tuesday.

With less than three weeks before the pandemic-postponed Games begin, concern is growing over a rebound in virus cases in Japan.

"In view of the current Covid-19 situation, it will be necessary to reduce the risk of infection by restricting the movement of members of the public," organisers said in a statement about the marathon and race walk events.

"It has therefore been decided to ask the public to refrain from spectating along the course."

The marathon was at the centre of controversy in the original build-up to the 2020 Games, when it was moved to northern Sapporo from Tokyo to avoid the capital's punishing summer heat.

The women's and men's marathon finals are among the last events at this summer's Games and will take place on August 7 and 8.

At a marathon rehearsal event in Sapporo in May, security guards stood with signs around their necks asking people to "please refrain from watching the race" to prevent infections.

That event had unfolded in a muted atmosphere, with scattered onlookers in facemasks clapping but refraining from cheering, to avoid spreading droplets.

Organisers on Tuesday vowed to work with local authorities "to ensure a safe and secure Tokyo 2020 Games for all participants and for the citizens of Sapporo and Hokkaido".

But with Covid-19 cases on the rise and fears over the spread of new coronavirus variants, Japan's government is this week expected to extend virus restrictions in several parts of the country.

The current measures, in place in Tokyo and Sapporo, cap spectators at sports events at 5,000 and limit the opening hours of bars and restaurants.

Japan's Covid-19 outbreak has not been as severe as in some countries, with around 14,800 deaths, but experts say another wave could stretch medical services as the Olympics begin.

For ticketed Olympic events, Games organisers last month set a limit of 10,000 domestic fans, or half of each venue's capacity. Overseas fans have already been barred.

But a rise in infections has forced a rethink -- with Games president Seiko Hashimoto recently warning that a closed-door Olympics remains an option.

Participants have already begun arriving in Japan, with 11,000 Olympic athletes from around 200 countries set to take part.

On Monday, a Japanese newspaper reported that fans would likely be banned from the July 23 opening ceremony over virus fears, but a reduced number of VIPs and Olympic officials would be able to attend.