North Korea on Thursday called South Korea's claim of an artillery exchange between the rivals "preposterous," saying a frightened South mistook construction noise for artillery when it accused Pyongyang of opening fire near the rivals' disputed maritime line.

North Korea said in a statement that the South overreacted to "normal blasting" from a North Korean construction project "aimed at improving the standard of people's living."

South Korean defence officials say marines returned fire Wednesday after North Korea launched artillery shells into the same waters that saw a deadly artillery exchange between the countries last November.

"It was preposterous in the age of science when latest detecting and intelligence means are available that they mistook the blasting for shelling," an unnamed North Korean official said in a statement released by the official Korean Central News Agency.

"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 Ministry spokesman Kim Min-seok said three North Korean shells fired near the Northern Limit Line in the Yellow Sea prompted the South to fire three shells back. Another ministry official, who refused to be named because of office policy, said North Korea fired more rounds later in the day and South Korea again responded.

All the shells landed in the water, South Korea said, and there were no reports of casualties.

High alert

South Korean forces have been on high alert in the area since a North Korean artillery attack killed four people in November on South Korea's Yeonpyeong Island. Wednesday's artillery exchange, which happened in hazy weather, was near that island, South Korea said.

It follows a recent easing of animosity between the Koreas and comes ahead of joint U.S.-South Korean military drills set for next week. Last month, a senior North Korean diplomat met with U.S. officials in New York to negotiate ways to restart long-stalled international talks aimed at persuading the North to abandon its nuclear weapons aspirations. The meeting came after the Koreas' nuclear envoys held cordial talks during a regional security forum in Indonesia.

On Wednesday, the United States appeared keen to get beyond the firing incident. State Department spokeswoman Victoria Nuland urged North Korea to exercise restraint and take steps to allow the six-nation disarmament talks to resume.

Violence often erupts in the contested slice of sea. Boats routinely jostle for position during crab-catching season, and three deadly naval clashes since 1999 have taken a few dozen lives.

Point of dispute

The maritime line separating the countries was drawn by the U.S.-led UN Command without Pyongyang's consent at the close of the 1950-53 Korean War, which ended with a truce, not a peace treaty, leaving the peninsula still technically in a state of war. The line is still a fierce point of dispute.

North Korea argues that the line should run farther south. Seoul believes accepting such a line would endanger fishing around five South Korean islands and hamper access to its port at Incheon.

The November attack marked a new level of hostility along the contested line. Two South Korean civilians and two marines died, and many houses were gutted in the shelling.

Baek Seung-joo, a military analyst at the state-run Korea Institute for Defence Analyses in South Korea, said the North appears to be rattling its sabres ahead of annual U.S.-South Korean military exercises planned for next week.