Japan is considering requiring Tokyo Olympic spectators to present proof of having tested negative for COVID-19 when entering a venue as part of measures to prevent the spread of the coronavirus, sources familiar with the matter said Monday.

The sources, who spoke on condition of anonymity, said some government officials have also proposed that spectators be allowed into venues if they show a certificate proving they have been vaccinated for the virus.

The government and other organizers of this summer's Olympics and Paralympics have already decided to bar spectators from abroad and aim to decide by the end of June how many domestic fans will be allowed to attend events.

The government and the organizers will also look into the option of only requiring temperature checks before admitting them into venues, a procedure already implemented at many sporting events in Japan held in front of a live audience, the sources said.

Despite opposition to holding the games this summer among a majority of Japanese people concerned about surging coronavirus infections, the organizers have insisted that the global sporting event can take place safely if adequate measures are taken.

The government on Friday extended the COVID-19 state of emergency in Tokyo, Osaka and seven other prefectures by three weeks to June 20 -- just over a month before the Olympics open in the capital -- as the decline in new infections is slow.

While Japan's vaccination rate has lagged well behind other developed countries, Prime Minister Yoshihide Suga, speaking at a press conference following his decision on the extension, sidestepped questions regarding whether the Olympics can be held under a state of emergency.

Suga said only that the government will "carry on with preparations while listening to a range of opinions," promising to take necessary measures against the virus, including keeping athletes and staff from coming in direct contact with the public and testing them regularly.