"There are no signs of peculiar movements," South Korean defense ministry deputy spokesman Na Seung-yong told a briefing. Na said a no-sail warning had not been sent to Seoul or the International Maritime Organisation (IMO).
North Korea has reported to the IMO before previous long-range missile launches, which it has claimed were rockets to launch satellites. The North is under U.N. sanctions banning it from developing ballistic missile technologies.
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U.S. Defense Secretary Ashton Carter is due to start a three-day visit to South Korea on Thursday.
South Korea's Yonhap News Agency quoted unidentified government officials as saying the no-sail warning has been in effect since April 1 and could indicate that a launch of a mid-range Rodong missile was "possible", according to an official quoted by Yonghap.
North Korea last test-fired its mid-range Rodong missile, which has a design range of 1,300 km (800 miles), in March 2014, while the leaders of South Korea, Japan and the United States were meeting to discuss the threat from the North.
Pyongyang did not issue a no-sail warning before that launch.
North Korea frequently test fires short-range missiles into the sea, often in what are seen as a response to the U.S.-South Korean drills, which it denounces as a preparation for war.
Last month, North Korea tested two short-range missiles off its eastern coast without designating a no-sail zone, drawing protest from Japan.
North Korea, which has threatened to carry out a fourth nuclear test, could be close to being able to put a nuclear warhead on a missile, some experts say, with the mid-range Rodong the most likely to be used.
Its leader, Kim Jong Un, visited one of its navy units, supervising torpedo attack drills, state media said on Saturday.