Tokyo was considering deploying several destroyers and patrol aircraft to the two areas to conduct warning and surveillance activities, the Yomiuri Shimbun said.
Japan would monitor the Tsushima Strait between its western coast and South Korea, and the area north-west of its southern island of Okinawa, the daily said, citing an outline of the government's plan.
According to the outline, Japan's maritime self-defence force would notify US and other forces deployed off North Korea's eastern and western coasts when it identified a suspicious ship, the report added.
Tokyo has been holding talks with Washington on how to inspect ships heading to and from North Korea after US Secretary of State Condoleezza Rice visited Japan last week.
Any decision on how far Japan is able to take part in the inspections is complicated by constitutional restraints on its military.
The post-World War Two constitution bans armed forces for offensive purposes but has been interpreted as allowing forces for self-defence.
In addition to surveillance, Japan is also studying the possibility of providing logistical support to US warships involved, including fuelling operations.
Japan responded swiftly to North Korea's nuclear test on Oct. 9, imposing sanctions that included a six-month ban on North Korean ships entering Japanese ports.
The government is looking to introduce another resolution to the UN Security Council that features a complete ban on imports from North Korea if Pyongyang conducts a second nuclear test or launches missiles, Kyodo news agency reported late on Saturday.

Copyright Reuters, 2006