South Korean news outlets say a North Korean patrol vessel sailed within the vicinity of the maritime border that separates the two rivals.
Information obtained from South Korea's Joint Chiefs of Staff says the vessel sailed in close proximity to the Northern Limit Line early Tuesday morning, near the northeastern coast of Gangwon Province.
The JCS issued an order to reinforce patrols in the area, and all South Korean fishing vessels to evacuate.
The North Korean vessel did not enter South Korean territorial waters. An official with South Korea's Joint Chiefs of Staff says it appears the ship sailed into the area to enforce a ban on North Korean fishing vessels operating in the vicinity of the NLL.