Japan's coast guard on Tuesday began issuing warnings to ships in the area to be on the lookout for falling debris from North Korea's planned rocket launch.
"We are announcing by radio the expected time and places where falling objects could appear," coast guard spokesman Yoshiyuki Terakado said.
Coast guard officials will issue the warning every day in Japanese and English until the launch is confirmed, he said.
Poor but nuclear-armed North Korea attracted international condemnation after it announced a plan to launch a satellite sometime from April 12 to 16, to mark the centenary of the birth of late founding president Kim Il-Sung.
Pyongyang insists the launch is a peaceful space project but the United States and its allies view it as a disguised missile test in breach of UN resolutions.
Japan has deployed missile defence systems to intercept and destroy the rocket if it looks set to fall on the country, a move similar to measures the country took in 2009 before Pyongyang's last long-range rocket launch.