Typhoon Maysak is west of Japan, maintaining sustained winds of 100mph with higher gusts, the Japanese Meteorological Agency said.
Japan’s main southern island of Kyushu was warned of heavy rainfall, fierce winds and possible mudslides.
Maysak had earlier brought down trees and knocked out power to hundreds of homes when it crossed Okinawa on Tuesday.
Hundreds of flights were cancelled in South Korea ahead of Maysak’s arrival, while North Korea scrambled to protect its buildings, roads and crops from the second typhoon to hit their peninsula in as many weeks.
Maysak will pass south of South Korea’s southern resort island of Jeju before making landfall near the southern mainland port of Busan early on Thursday.
South Korea’s weather agency said the typhoon will affect most of the country and warned of potential damage from the “very strong winds and very hard rain”.
More than 340 domestic flights in and out of Jeju were cancelled as of Wednesday morning, the Korea Airports Corporation said.
Authorities are also restricting entry to public parks and moving hundreds of fishing boats and passenger vessels to safety.
North Korea’s official Korean Central News Agency (KCNA) said officials were working urgently to minimise damage from the typhoon, which was forecast to graze its eastern region before reaching land near the north-eastern city of Chongjin.
KCNA said officials are examining buildings, roads, railways, cropland and drainage systems and moving fishing boats to safety.
“Scrupulous” protective measures were being applied to power stations to ensure stable power supply during the storm, the report said.
Another storm in the Pacific is blowing north and forecast to affect North Asia by the weekend.
Tropical Storm Haishen, packing sustained winds of up to 56mph, could gain in ferocity before slamming into Japan’s southern main islands of Kyushu and Shikoku before reaching the Korean Peninsula on Monday.
Typhoon Bavi last week damaged homes, buildings and crops in the Koreas.
South Korean authorities said no casualties were reported, and North Korean state media did not mention any deaths or injuries.
