Tokyo Gov. Yuriko Koike issued a warning Tuesday amid signs of a possible resurgence of coronavirus infections, as 34 cases were newly reported in the Japanese capital, Kyodo News reported.

The warning she calls a "Tokyo alert" serves as a call for caution.

But if the number further increases, the Tokyo government plans to issue requests again to businesses and people that would halt social and economic activities.

The number is the highest since Japan fully lifted its state of emergency last week and the first time since May 14 for daily infection cases to top 30.

At a coronavirus task force meeting late Tuesday, Koike urged Tokyo residents to "be very cautious of going to places such as the nightlife districts" that put people at risk of being in closed spaces, crowded places and close contact with people.

She told reporters, "Among today's figures, I've received reports that there are many people" related to the nightlife industry especially in Shinjuku.

The alert, however, is not intended to move back the phases she has already implemented, according to Koike.

"The request to suspend business will be considered when there are more than 50 positive cases in a day and also evaluating other indexes," she said.

Tokyo has had the largest number of infections in Japan since late March, after which it reported its first three-digit increase of 116 on April 4. The figure has been gradually decreasing since mid-April, but this week Tokyo has seen a slight bump up.

Koike said, last month that Tokyo will issue the alert if the number of new infection cases exceeds 20 a day or the ratio of the weekly new infection increases to more than one compared with the previous week. Another yardstick would be if the ratio for unknown infection routes climbs to more than 50 per cent.

If the numbers exceed maximum-allowed levels on several indexes, other barometers will be taken into account.

The number of infected people in the past seven days compared with the previous seven days was more than double and the ratio for unknown infection routes was 50 per cent, which met the standard to issue the alert, a Tokyo government official said.

However, the daily average of newly confirmed cases in seven days was 16.3 people, which was below the criterion, the official said. - BERNAMA