SEOUL: North Korea responded to new UN sanctions with fresh threats of nuclear war Friday, vowing to scrap peace pacts and sever a hotline with South Korea as it upped the ante after its recent atomic test.

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

It has even threatened a "pre-emptive nuclear attack" against the United States and South Korea -- a notion dismissed as bluster by analysts, but not without dangerous, underlying intent.

North Korea "abrogates all agreements on non-aggression reached between the North and the South", the state-run Committee for the Peaceful Reunification of Korea (CPRK) said in the latest statement Friday.

The CPRK 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 notifies the South side that it will immediately cut off the North-South hotline," the committee said in a statement carried by the official Korean Central News Agency.

The hotline was installed in 1971 and the North has severed it on five occasions in the past -- most recently in 2010.

Pyongyang's latest announcement came hours after the UN Security Council beefed up existing sanctions on the communist state in response to its February 12 nuclear test.

The resolution adopted by the 15-member Council tightened restrictions on North Korea's financial dealings, notably its suspect "bulk cash" transfers.

The new sanctions will "bite hard", said the US ambassador to the UN, Susan Rice. "They increase North Korea's isolation and raise the cost to North Korea's leaders of defying the international community."

China wants "full implementation" of the resolution, said its UN envoy Li Baodong, while stressing that efforts must be made to bring North Korea back to negotiations.

Prior to the Security Council meeting, the North Korean foreign ministry had threatened a "pre-emptive nuclear attack" against the United States and all other "aggressors".

The United States responded by saying it was "fully capable" of defending itself and its allies -- including South Korea -- against any missile strike.

Friday's CPRK statement condemned the UN resolution as proof that Washington and its "puppets" in Seoul were "hell bent" on confrontation.

"North-South relations have gone so far beyond the danger line that they are no longer reparable and an extremely dangerous situation is prevailing... where a nuclear war may break out right now," it said.

An annual US-South Korea military exercise known as Foal Eagle is currently underway and another joint drill is scheduled to begin Monday.

The North is believed to be gearing up for nationwide military manoeuvres of its own next week, involving all three wings of its armed forces.

In such a volatile atmosphere, "there's always that risk of a miscalculation and rapid escalation", warned Dan Pinkston, a Seoul-based security expert for the International Crisis Group.

"Most of this is bluster, but the regime in North Korea is also signalling that it's willing to take greater risks, and that's a dangerous sign," Pinkston told AFP.

KCNA said North Korean leader Kim Jong-Un on Thursday visited a frontline military unit involved in the shelling of a South Korean island in 2010.

During his inspection, Kim declared the North was ready for "all-out war" and that he would order a frontline assault in response to any perceived aggression.

"To me, this feels like the most dangerous situation since the Korean War," said Paik Hak-Soon, a North Korean analyst at the Sejong Institute in Seoul who believes Pyongyang is determined to provoke a confrontation.

The North's rhetoric was at full pitch again at a massive rally of 100,000 troops and civilians in the centre of Pyongyang on Thursday.

Addressing the crowds, Vice Defence Minister Kang Pyo-Yong said the North had intercontinental ballistic missiles on standby, "loaded with light, miniatuarised nuclear warheads."