Japanese Prime Minister Yoshihide Suga said on Sunday that he would expand emergency measures as needed to contain a new wave of coronavirus infections, amid fears over the spread of virus mutations.
Suga, asked on a Fuji TV programme whether Tokyo might be added to a list of areas including the western metropolis of Osaka set to come under lockdown measures from Monday, said, "All possibilities are being considered."
"It doesn't matter specifically where, we will act without hesitation if needed," he said.
Japan is grappling with a COVID-19 resurgence ahead of the summer Olympics scheduled to begin in July, with large-scale vaccinations of the general population yet to begin.
On Sunday, 355 new infections were reported in Tokyo, although that is still well below the peak of over 2,500 in January.
Health experts have been particularly concerned about a surge in mutations among those who have recently tested positive around Osaka. The variant, known to have emerged in Britain, is feared to be highly transmissible.
A total of 594 new coronavirus cases were reported in Osaka prefecture on Sunday, a day after a record 666 were confirmed.
Variants of the virus have cropped up around the world since last year, including the E484 mutation detected in a growing number of cases in Tokyo, officials say.
Around 70% of coronavirus patients tested at a Tokyo hospital last month carried the E484K mutation, nicknamed "Eek" by some scientists and known for reducing vaccine protection, said public broadcaster NHK.
The "Eek" mutation was found in 10 of 14 people who tested positive for the virus at Tokyo Medical and Dental University Medical Hospital in March, the report said.
For the two months through March, 12 of 36 COVID patients carried the mutation, with none of them having recently travelled abroad or reporting contact with people who had, it said.
Hospital officials were not available to confirm the report, which said that none of the patients there carried the British strain.