Japan has detected radio signals suggesting North Korea may be preparing another ballistic missile launch in the coming days, although such signals are not unusual and satellite images did not show fresh activity, government sources said Tuesday.

After firing missiles at a pace of about two or three a month since April, North Korean launches paused in September, after it fired an intermediate-range missile over Hokkaido.

"This is not enough to determine (if a launch is likely soon)," one source said.