"It would be a matter of time before Tokyo faces a situation similar to Osaka," Koike said.
She said timing and details of the new measures, including shorter hours for restaurants and bars, will be decided later.
Later Thursday, Prime Minister Yoshihide Suga said he plans to place Tokyo under emergency virus measures after consulting with experts on Friday. Suga said he is also discussing the possibility of similar steps with other prefectures where infections have spiked.
The latest surge started mainly in western Japan, including Osaka, where the daily toll hit a record 905 and Gov. Hirofumi Yoshimura asked that the Olympic torch relay scheduled for Tuesday and Wednesday be held at a park and not on the public road. He said more than 70% of hospital beds have been occupied, a threshold for a local medical alert.
Infections have also surged elsewhere in the country. With Japan's vaccination drive still at an early stage, the surge may cause further cancelations of Olympic-related events.
Kyodo News agency on Wednesday reported that Japan is considering prioritizing its Olympic athletes for vaccination by late June. Chief Cabinet Secretary Katsunobu Kato on Thursday denied that his government is discussing any details at the moment, but added that he is closely watching discussions among Olympic officials about how to ensure athletes' safety.
Inoculations started mid-February with medical workers, but still account for less than 1% of the population. Vaccinations of elderly people are to start next week and the rest will likely have to wait until around July.
Though Japan is not doing as well as some other Asian countries in controlling the virus, its cases and deaths are much smaller than in the United States and Europe even though it has never imposed a hard lockdown. As of Wednesday, Japan had 492,875 cases and 9,301 deaths, according to the health ministry.
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This story corrects when Tokyo's state of emergency was lifted.