South Korea has said any threat of a nuclear attack made by North Korea would inevitably signify its demise.

North Korea has said that it was cancelling a hotline and a non-aggression pact with rival South Korea whilst, the United Nations imposed tough, new sanctions targeting its economy and leadership.

The new UN sanctions came in response to North Korea's underground nuclear test on 12 February and were the fourth set imposed by the UN since the country's first test in 2006.

They are aimed at reining in Pyongyang's nuclear and missile development by requiring all countries to freeze financial transactions or services that could contribute to the programs.

North Korea also said it was voiding past nuclear disarmament agreements between North and South Korea.

Meanwhile, the South Korean Defence Ministry spokesman, Kim Min-seok, said the North's threat of a nuclear attack would doom its regime.

North Korea's Committee for the Peaceful Reunification of Korea, the country's arm for dealing with cross-border affairs with Seoul, said it will retaliate with "crushing strikes" if enemies intrude into its territory "even an inch, and fire even a single shell."

Peace activists gathered in front of the US Embassy in Seoul today to denounce the sanctions.

Activists believe they will only worsen the already tense situation in the Korean Peninsula, and insisted the UN Security Council to remove such resolutions.

South and North Korea agreed in a 1992 joint declaration not to produce, test or use nuclear weapons. North Korea has since conducted three nuclear tests.

The resolution also targets North Korea's ruling elite by banning all nations from exporting expensive jewelry, yachts, luxury automobiles and race cars to the North.

It also imposes new travel sanctions that would require countries to expel agents working for sanctioned North Korean companies.

The success of the sanctions could depend on how well they are enforced by China, where most of the companies and banks that North Korea is believed to work with are based.

Tensions with North Korea have escalated since Pyongyang launched a rocket in December and conducted last month's nuclear test - the first since Kim took charge.

Many countries, especially in the region, had hoped he would steer the country toward engagement and resolution of the dispute over its nuclear and missile programs.