Seoul - North Korea warned Friday it would use "merciless" military action to defend its sea border with South Korea.

In a statement carried by the state-run Korean Central News Agency, North Korea's military demanded an apology for a naval clash earlier this week in the Yellow Sea and said it would "take merciless military measures" to defend its unilaterally drawn maritime border.

North and South Korean navy vessels were involved in a skirmish near the Northern Limit Line, a de-facto maritime border, off their west coast earlier this week. Seoul says a northern patrol boat crossed the line and opened fire at its ships after they fired warning shots, while Pyongyang says southern warships violated its waters.

According to the South Korean military, the northern vessel was heavily damaged.

North Korea does not recognize the Northern Limit Line, which was drawn by the US after the 1950-53 Korean War, but instead regards an extension of the Military Demarcation Line as its border with the South.

Seoul was accused of a shameless, politcally-motivated provocation and Pyongyang warned Thursday that the South's army would have to "pay dearly" for the clash.

A South Korean official, who was quoted by the Yonhap news agency, however, dismissed Pyongyang's latest threats as "rhetoric."

Pyongyang was attempting to cast the blame for the incident on the South, said the official, who spoke on condition of anonymity.

"We're seeing no particular movement with the North Korean military," he added.

Lee Jong Hoo, a spokeswoman for the Unification Ministry in Seoul, also dismissed the threat, saying it was rhetoric "North Korea customarily uses" but has not put into action.

The rise in inter-Korean tension came shortly before US President Barack Obama is scheduled to visit South Korea on his first Asia trip. North Korea's nuclear weapons programme is expected to top the agenda of Obama's visit. (dpa)