North Korea says it will scrap peace pacts with South Korea and have threatened nuclear action following the new UN sanctions placed on the country.

The United States warned that the comments were "not helpful" and said the latest threats would not improve the lot of the North Korean people.

Pyongyang is renowned for its bellicose rhetoric, but the tone has reached a frenzied pitch in recent days, fuelling concerns of a border clash with both North and South planning major military exercises next week.

It has also threatened a "pre-emptive nuclear attack" against the US and South Korea -- a notion dismissed as bluster by analysts.

A US state department spokeswoman added the comments "are just not going to improve conditions for the North Korean people".

The Committee for the Peaceful Reunification of Korea said the pacts would be voided as of Monday, the same day that Pyongyang has vowed to rip up the 1953 armistice agreement that ended Korean War hostilities.

It also announced the immediate severing of a North-South hotline installed in 1971.