Japan has warned that coronavirus infections are surging at an unprecedented pace as new cases hit a record high in Tokyo, overshadowing the Olympics and adding to doubts over the government's handling of the pandemic.

The Delta variant was leading to a spread of infections "unseen in the past", Health Minister Norihisa Tamura said as he defended a new policy of asking patients with milder symptoms to isolate themselves at home rather than going to the hospital.

"The pandemic has entered a new phase ... Unless we have enough beds, we can't bring people to the hospital. We're acting pre-emptively on this front," Tamura told parliament.

Japan has seen a sharp increase in coronavirus cases with Tokyo reporting a record 4,166 new cases in the last 24 hours.

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