Pyongyang said Seoul's reaction was "preposterous" and that the sounds were those of "normal blasting" at a
building site near the maritime border.
Seoul earlier said it fired warning shots after the North's shelling off the South's Yeonpyeong island.
Last November the North fired shells at Yeonpyeong, killing four people.
'Tragicomedy'
On Thursday, North Korea's official KCNA news agency accused Seoul of overreacting to "normal blasting" from a North Korean construction project "aimed at improving the standard of people's living".
"It was preposterous in the age of science when latest detecting and intelligence means are available that they mistook the blasting for shelling," the KCNA report said.
"It was a tragicomedy that they indiscriminately reacted to what happened with counter-shelling even without confirming the truth about the case in the sensitive waters."
South Korean defence officials earlier said there were two separate shelling incidents on Wednesday. All the shells landed in the water
The South's decision to reply with warning shots follows strong criticism for failing to react with force to the attack on Yeonpyeong last year.
Pyongyang said at the time that it had been provoked by military exercises being held by the South close to the island.
The incident triggered outrage in South Korea, which increased troop numbers on Yeonpyeong and said it would respond more forcefully in the future.
The crisis came months after the sinking of a South Korean warship, the Cheonan, widely blamed on North Korea.
