Japan's southwestern Okinawa Prefecture has issued a warning as influenza case numbers shoot up nationwide.

The National Institute of Infectious Diseases says that about 5,000 medical institutions nationwide reported 36,388 cases of seasonal flu during the week through January 15. The figure was more than 1.5 times that of the previous week.

The tally of influenza patients across the country is estimated to stand at about 257,000.

In Okinawa, the number of flu patients per institution during the seven-day period was 33.23, prompting the prefectural government to issue a warning on Thursday.

The number was above 10 in seven other prefectures, including Fukuoka Prefecture, where the figure stood at 16.96.

Toho University Professor Tateda Kazuhiro says that some areas are struggling with both coronavirus and seasonal flu outbreaks, and called on the public to make every effort to protect themselves from infection.

Meanwhile, Japan's health ministry confirmed 82,614 new cases of coronavirus infection and 425 deaths nationwide on Friday. The number of seriously ill patients who are on ventilators or ECMO heart-lung machines stood at 673, down eight from Thursday.

In Tokyo, the number of new infections was 6,603, down 4,638 from a week before.