North Korea has deployed a new generation of high-speed, radar-evading warship designed to infiltrate special forces into South Korean territory.
The Very Slender Vessel sits low in the water and is designed to pierce waves instead of riding over them.
Capable of travelling at nearly 60 mph, the craft are armed with heavy machine guns and torpedoes, with the JoongAng Ilbo quoting South Korean military officials as saying the ships pose a "new threat" to its security.
South Korea first learned that its neighbour was developing VSVs late last year when satellites caught images of early tests with the new technology in the East Sea.
Intelligence officials in Seoul now estimate that at least seven of the vessels have been put into service on the west coast of the peninsula, with one of the craft recently sighted off Yongmae Island.
The island is just seven miles from the South's Yeonpyeong Island, which North Korean artillery batteries bombarded in November 2012, killing two civilian residents and two military personnel.