North Korea may attempt a third spy satellite launch as early as midnight on Wednesday, Japanese media reported.
Tokyo said Pyongyang has notified them of a nine-day window for the launch, which closes at 23:59 local time (14:59 GMT) on 30 November.
Japan will work with South Korea to "strongly urge" the North not to proceed with the launch, which they said would violate UN resolutions.
Pyongyang has twice failed this year to get a spy satellite into space.
The Japan Coast Guard said Pyongyang's notification designated three maritime zones believed to be the areas where debris from the rocket carrying the satellite will fall.
Two are to the west of the Korean Peninsula and the other is to the east of the Philippines' island of Luzon.
Kang Ho-pil, chief director of operations at the South Korean Joint Chiefs of Staff, warned that Seoul will take "necessary measures" should the launch proceed.
A spy satellite is a coveted prize for North Korean leader Kim Jong Un, as it will allow him to monitor incoming attacks and plot his own more accurately.
However the United Nations Security Council has banned Pyongyang from launching satellites because it sees them as a pretext to test the North's missile technology.
South Korea retrieved debris from the North's first launch in May and said the satellite had "no military utility". After a second attempt in August failed, Pyongyang's space agency said it would try again in October but did not do so.
Russian President Vladimir Putin suggested in September after a meeting with Mr Kim that Moscow could help Pyongyang build satellites, but the details of what was actually promised are unclear.
Earlier this month, South Korea announced plans to launch its own spy satellite by the end of November. The satellite is to be carried by a rocket from US company SpaceX.
This is reportedly the first of five spy satellites Seoul plans to launch into space by 2025.
