"There is neither way to improve [relations] nor hope to bring them on track." 

But Kim Ho-nyeon, a spokesman for South Korea's unification ministry, said "agreements between the South and the North cannot be scrapped unilaterally".

"The confrontation between the North and the South in the political and military fields has been put to such extremes that inter-Korean relations have reached the brink of a war."

North Korean statement.

"We urge North Korea to come forward to dialogue."

The North's statement also criticised the government of Lee Myung-bak, the South Korean president, for "ruthlessly scrapping" pacts reached at summits in 2000 and 2007.

The North has never recognised the Northern Limit Line (NLL), a sea border drawn unilaterally by US-led United Nations forces after the 1950-1953 Korean war.

Although the line has served as a de facto border, North Korea has frequently demanded it be redrawn - a move the South has rejected.

Six South Korean soldiers were killed in a naval clash in June 2002 in the area, while the North's casualties were said to be heavier.

In 1999, a similar clash killed dozens of North Korean sailors.

'Firm response'

Reacting to the North Korean announcement, military officials in the South vowed a firm response to any North Korean violation of the sea border.

 North Korea has accused the South of pushing for confrontation [EPA]
"We will respond firmly if the NLL, which has been serving as a de facto sea border for the past five decades, is breached," the defence ministry said in a statement on Friday.

"We will uphold the maritime border just as we maintain the military demarcation line on land," Won Tae-Jae, a ministry spokesman, told reporters.

"The agreement reached between the Koreas cannot be scrapped just because one side decides to scrap it."

The North and South have remained technically at war because the 1950-53 Korean War ended without a peace treaty, although the two countries have signed various agreements over the years.

A series of deals were reached during a brief period of calm that followed a landmark North-South summit in June 2000, which led to reunions of separated families, as well as the introduction of communication systems to defuse military tensions and rail and road links across their heavily-armed border.

But late last year, with relations rapidly deteriorating, Pyongyang announced it was closing most border traffic between the two states and suspending several cooperation projects.

Both sides have deployed thousands of soldiers along the Demilitarised Zone (DMZ) that divides the two countries, with almost 30,000 US military personnel based in South Korea as well.

Political manoeuvring

North and South Korea have never signed a peace treaty ending the Korean War [Reuters]
Some analysts have said they believe the North is raising tensions to ensure it remains a diplomatic priority for the new Obama administration in the US.

Yoo Ho-yeol, a professor at Korea University told the Associated Press the latest comments had three main aims - to pressure the South Korean president, scare the United States and to drum up political support at home.

"The North probably believes that this type of thing is the most effective way of getting the upper hand with the US ahead of negotiations by raising tension," he said.

"What is worrying is that the possibility of a military clash is rising."

North Korea has labelled the policies of Lee as "confrontational", primarily in reaction to his decision to end the flow of unconditional aid to the impoverished country.

Seoul has promised massive aid and investment only if Pyongyang takes concrete measures to show it is serious about giving up its nuclear weapons programme.

North Korea tested a nuclear weapon in 2006. It then signed an accord a few months later with the US, China, Japan, South Korea and Russia under which it agreed to dismantle its nuclear programme in exchange for aid and other concessions.

However, the North continues to be at odds with demands from the US and other parties that it must open its nuclear facilities to external inspection in order to verify its disarmament.