North Korea has declared a no-sail zone off its east coast as tensions continue to grow in the region.
Pyongyang gave the warning after it carried out live-fire drills which have been condemned as provocative.
The drills triggered a response from South Korea, which opened fire after shells crossed the Northern Limit Line (NLL), the disputed maritime border between the two Koreas.
South Korea's vice-defence ministry spokesperson Wi Wong-seop said: "We consider that the North's announcement contains hostile intentions. [North Korea] is believed to be creating a crisis situation on the Korean peninsula by raising tensions near the western maritime border."
A military spokesman told Yonhap News agency: "North Korea has internally set the no-sail zone in waters near Wonsan to control its vessels".
Following the warning, vessel activity monitored by the NK News DPRK ship tracker showed reduced traffic in the coastal areas to the immediate east and west of North Korea.
North Korea's foreign ministry condemned UN Security Council criticism of the fire drills.The ministry added that Pyongyang would "categorically" reject any subsequent UN sanctions as a breach of sovereignty.
According to a North Korean government source: "[The fire drills are] justifiable self-defence behaviour for us to conduct to preserve peace in the region and to protect the safety of our people and our country".