Japanese Prime Minister Shinzo Abe has urged people in the southern islands to exercise the utmost caution as Typhoon Haishen blows in.
The storm is pummelling the region with high winds and drenching them with heavy rain, knocking out power to tens of thousands of homes.
Some eight million people were asked to evacuate in the path of the storm.
After passing over Japan, Haishen is expected to make landfall on Monday in South Korea, which is also on alert.
While it weakened and shifted slightly as it approached land, Haishen remained “large” and “extremely strong”, the director of the Japan Meteorological Agency, Yoshihisa Nakamoto, was quoted as saying by AFP news agency.
“Record-level rainfall is expected,” he said. “It may cause landslides or it could cause even large rivers to flood.”
Surging tides, he added, could cause widespread flooding in low-lying areas, particularly around river mouths.
Speaking at a meeting with cabinet ministers, Prime Minister Abe issued an appeal.
“To all citizens,” he said, “especially those who are living in areas which have a high possibility of river flooding or high tides, please stay alert to the information from your local authorities. And please take immediate action, to ensure your safety, to protect your life.”
The typhoon has also forced Japan’s coast guard to suspend its search for missing sailors from a cargo ship that sank during Typhoon Maysak. The Gulf Livestock 1 was carrying 43 crew members and 6,000 cows when it went missing on Wednesday. Three crew members were rescued alive.
How is South Korea preparing?
The Central Disaster and Safety Countermeasure Headquarters raised the country’s typhoon warning level to four – the highest – at 19:00 (10:00 GMT) on Sunday.
The Korea Forest Service also raised the landslide alert to its highest level.
Its chief, Park Chong-ho, told Yonhap news agency: “Huge damage is expected as the typhoon this time is forecast to be more dangerous than the previous ones that affected South Korea shortly after the end of the monsoon season.”
Typhoon Haishen is forecast to move close to Busan on Monday morning.