A North Korean coastal artillery battery fired three shells into waters near the Northern Limit Line northeast of Yeonpyeong Island in the West Sea on Wednesday afternoon. The South Korean military fired three K-9 self-propelled howitzer shells back.

"We heard the blasts of three shells presumed to have been fired from the North Korean military's coastal artillery battery into waters south of the North's Yongmae Island, which lies northeast of Yeonpyeong Island, around 1 p.m.," an officer with the Joint Chiefs of Staff said. "Presuming that one of them fell into waters near the NLL, we fired three K-9 self-propelled howitzer shells from Yeonpyeong Island into waters near the NLL in response an hour later."

Since only one of the North Korean shells fell in South Korean waters, the Marine Corps' Yeonpyeong unit fired three shells under new operational guidelines stipulating that return fire should be at least three times more powerful than the attack.

The military warned the North of the impending response to the provocation through an international merchant marine communication channel around 1:25 p.m. and also warned residents on Yeonpyeong Island to prepare for evacuation.

The North fired the shells from a 130 mm coastal artillery piece with a maximum range of 27 km or a 76.2 mm gun with a maximum range of 12 km, Seoul said. Yongmae lies about 11 km from the NLL and 19 km from Yeonpyeong.

In the evening, the North Korean coastal artillery battery fired three additional shells and one fell into waters near south of the NLL. The Marine Corps fired back three shells again, this time 15 minutes later.

Asked why it took almost an hour to return fire in the first instance, military authorities said that no direct damage was done and it took some time to check whether the North Korean shells had crossed the NLL and analyze the movement's of the North Korean military.

But critics said the response took much longer than expected, given that the military promised to react immediately to any North Korean provocations after its failure to respond properly to attacks on the Navy corvette Cheonan and Yeonpyeong Island last year.

Military authorities believe the provocation was an expression of pique ahead of joint South Korea-U.S. military exercises that kick off next Tuesday and aimed to discover how quickly and resolutely the South Korean military could respond.