Typhoon Jongdari, meaning "skylark" in Korean, will strengthen to a category four typhoon as it moves northwest but will be off peak strength as it draws closest to Japan's largest main island of Honshu late on Saturday or early on Sunday.

While the heaviest rains are likely to hit the region around Tokyo, with 300-500mm possible in the 24 hours to Sunday morning, the storm could also pass right over the western region lashed by floods earlier this month, which resulted in the loss of at least 219 lives.

"There's not much we can do, it's a natural phenomenon," a man in a flood-hit area of Okayama prefecture who was cleaning his damaged home told NTV.

"I really do wish it wasn't heading here, though."

Heavy rains and winds were already lashing the Ogasawara islands, about 1000km south of Tokyo, by Friday afternoon as the storm - bracketed by two high pressure areas - churned towards Japan on an unusual northwest course.

July has seen Japan hit by one weather disaster after another, including a record-breaking heatwave in which temperatures surged to 41.1C near Tokyo on Monday and has killed at least 80 people, with more than 20,000 taken to hospital for treatment.

Australian Associated Press