SEOUL - The Republic of Korea (ROK) has fired artillery shells into the the Democratic People's Republic of Korea (DPRK) waters in response to North Korean shells from a live-fire drill that fell in waters south of the rivals' disputed western sea boundary.

An official with the ROK's Joint Chiefs of Staff says that no DPRK shells hit any land or military installations Monday. He says that the ROK responded with artillery fire. He gave no details and spoke on condition of anonymity because of office rules.

The ROK also evacuated border island residents amid DPRK live-fire drill, Xinhua reported.

The exchange of fire follows Pyongyang's earlier, unusual announcement that it would conduct the drills, a move seen as an expression of Pyongyang's frustration at making little progress in its recent push to win outside aid.

Pyongyang also threatened to conduct a fourth nuclear test, though Seoul says it isn't imminent.