South Korea's Yonhap news agency said on Tuesday North Korea may be preparing to launch more short-range missiles.
The report said launches are coming from the western part of the Korean peninsula, quoting a government source.
North Korea has issued a warning for vessels to stay out of waters off its coasts, according to Japan's coast guard and Yonhap.
North Korea has well over 1,000 missiles of various ranges. It has sold missiles and missile technology overseas, with Iran being one of the large buyers.
A U.N. resolution bans it from launching ballistic missiles.
The North has more than 800 ballistic missiles -- including 600 Scud missiles of various types and 200 Rodong missiles.