A South Korean man at a railway station in Seoul watches footage Tuesday of South Korean sailors in a 2002 clash with the North Korean navy. (Ahn Young-joon/Associated Press)

North Korea intensified its rhetoric against South Korea on Thursday, just days after their two navies exchanged gunfire in disputed waters on the west coast of the Korean Peninsula.

North Korea's main Rodong Sinmun newspaper denounced South Korea and said aggression would not be tolerated.

"Our unchanged principle is no forgiveness and merciless punishment for warmongers who infringe upon our republic's dignity and sovereignty," the newspaper said in a commentary that was carried by the official Korean Central News Agency.

The commentary did not carry any specific threats.

Another state-run newspaper, Minju Joson, said that South Korea would face "costly consequences." The newspaper claimed the skirmish was part of a South Korean plot to disrupt planned talks between the North and the United States.

On Tuesday, a North Korean warship was heavily damaged when the navies of the two Koreas briefly exchanged fire, South Korean officials said.

The incident took place about 220 kilometres off the South Korean port city of Incheon, near the island of Daecheong, which is controlled by South Korea.

The sea boundary on the west coast has been disputed by North Korea since it was drawn by the United Nations commander in Korea at the end of fighting in the Korean War in 1953.

North Korea wants the line moved farther south.

Southern rebuff

South Korean officials sloughed off the North's threats.

"We will resolutely safeguard" the Northern Limit Line, the name of the disputed sea borderline, an unnamed Defence Ministry official said.

A South Korean academic said the North may have initiated the skirmish to increase tensions ahead of meetings between North Korea and Washington.

Yang Moo-jin, a professor at Seoul's University of North Korean Studies, suggested it was in the North's interest to "show the Korean Peninsula is still unstable" ahead of the talks.

The two Koreas are still technically at war, since the Korean War ended without a peace treaty.