Tokyo reported 206 novel coronavirus infections Sunday, marking a record fourth straight day of more than 200 new cases, as signs of a resurgence of the pandemic were also seen in neighboring prefectures and Osaka.

Tokyo's single-day figure, the same as on Saturday, was reported amid increasing concern among the public as well as authorities about a potential second wave, given that infection routes for nearly half of the total positive cases confirmed Sunday were unknown.

As of Sunday evening, another 409 cases had been reported across Japan, with Chiba and Osaka prefectures seeing their highest figures since a state of emergency was lifted in May.

Tokyo's cumulative total reached 7,927, accounting for nearly a third of the around 22,600 confirmed cases in the country of COVID-19, the respiratory illness caused by the virus, including 712 from the Diamond Princess cruise ship that was quarantined in Yokohama near Tokyo in February.

Despite the uptick, none of those confirmed by the Tokyo metropolitan government as infected Sunday, as well as in recent days, were in serious condition, with roughly 70 percent in their 30s or younger.

In Tokyo, 580 COVID-19 patients are currently hospitalized and five of them are in serious condition, according to the metropolitan government.

The number of cases has been on an upward trend since the nationwide state of emergency was completely lifted on May 25, with infections spreading gradually from Tokyo to nearby prefectures.

Chiba Prefecture, east of the capital, reported 31 new cases, the first time since April 18 that its single-day figure had topped 30.

Kanagawa Prefecture, which includes major cities such as Yokohama and Kawasaki on the outskirts of the capital, reported 34 cases Saturday, its highest figure since the end of the state of emergency. On Sunday, 24 cases were reported.

Yukio Edano, leader of the main opposition Constitutional Democratic Party of Japan, urged the central government to declare a state of emergency for Tokyo and surrounding areas.

"It's unacceptable to leave the situation without doing anything," Edano told reporters in the city of Chiba.

The western Japan prefecture of Osaka reported 32 cases, prompting Gov. Hirofumi Yoshimura to issue a "yellow" alert based on its own criteria for gauging the seriousness of the pandemic.

Yoshimura warned the virus has been spreading in "so-called nightlife districts among young people."

Tsutenkaku Tower in Osaka lit up in yellow on July 12, 2020. (Kyodo)