Further questions are being raised about this summer's Olympics with Japan set to declare a state of emergency in Tokyo as cases of Covid-19 surge.

With thousands of new cases resulting from highly infectious strains of the virus, Prime Minister Yoshihide Suga said the government wanted to decide this week whether to declare the state of emergency for major parts of the country.

Suga said the capital Tokyo was mulling a request to the central government to issue the state of emergency, as Osaka and Hyogo prefectures already have done.

Japan has so far avoided an explosive spread of the pandemic that has plagued many Western countries, with total cases so far at about 540,000 and a death toll of 9,707. But the latest rise in infections has stoked alarm, coming just three months before the planned start of the Tokyo Olympics and amid a sluggish vaccination roll-out.

On Wednesday Tokyo reported 843 new infections, the most since the end of January when its previous state of emergency was in place. Case numbers in Osaka have exceeded those in Tokyo in recent days, reaching a record 1,351 on 13 April.

Tokyo Governor Yuriko Koike is preparing to request an emergency period be declared from 29 April to 9 May, encompassing Japan's annual 'Golden Week' holiday period, the Mainichi newspaper reported.

Kyoto prefecture is also preparing to request an emergency declaration, Japan's top government spokesman Katsunobu Kato said.

If adopted in all four regions from Tokyo to Kyoto, the emergency measures would cover close to a quarter of Japan's population of 126 million.

Earlier on Tuesday, Suga claimed that any emergency declaration would have no impact on the Olympics scheduled to begin in July.

Kato, the top government spokesman, also said Wednesday the government would continue to work for a "safe and secure" Tokyo Olympics.

Meanwhile Pfizer Inc will sign a contract this month to supply an additional 50 million doses of vaccine to Japan by September, the Nikkei newspaper reported.