An enraged North Korea responded to new UN sanctions with fresh threats of nuclear war on Friday, vowing to scrap peace pacts with South Korea as it upped the ante yet again after its recent atomic test.

The United States warned the comments were "not helpful" and said tearing up peace agreements unilaterally would carry no legal weight.

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 even threatened a "pre-emptive nuclear attack" against the US 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 Friday.

Responding, White House spokesman Joshua Earnest said: "North Korea's threats are not helpful. We have consistently called on North Korea to improve its relations with its neighbours, including South Korea."

"As a legal matter, no party to the armistice can unilaterally terminate or alter the terms of the Armistice Agreement," Pentagon spokesman George Little said in an email.

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

State television, meanwhile, showed North Korean leader Kim Jong-Un laying preparations for "all-out war" as he visited a frontline military unit involved in the shelling of a South Korean island in 2010.

Footage of the visit showed him being greeted by chanting troops who were held back as they surged towards him. Their families brought children to meet the leader, with one woman encouraging her daughter forward for a hug.

At the end of the trip, the soldiers ran down to the beach and waded chest deep into the freezing water clutching at Kim's motor launch as it moved away.

The November 2010 shelling came eight months after the sinking of a South Korean naval vessel with the loss of 46 lives that was also blamed on Pyongyang.

While North Korea is not deemed capable of any nuclear strike on the US mainland, there are growing fears that it will mount some provocation in the form of a missile test or a similar artillery assault.

"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.

"The North is cornered more than ever in the international community and will keep pushing ahead with even more confrontational moves militarily," Paik told AFP.

South Korea's new president, Park Geun-Hye, who was sworn in less than two weeks ago, said the situation had become "very grave" but vowed to "deal strongly" with any provocation from the North.

The CPRK statement came hours after the UN Security Council beefed up existing sanctions on the communist state in response to its February 12 nuclear test.

-- Sanctions will 'bite hard' --

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."

Germany's foreign minister said he wanted his EU counterparts to consider further measures against North Korea beyond UN sanctions, while France urged Pyongyang to show restraint.

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

In Beijing, foreign ministry spokeswoman Hua Chunying urged "relevant parties to exercise calm and restraint, and avoid actions that might further escalate tensions," describing the situation as "highly complex and sensitive".

Prior to the Security Council meeting, the North Korean foreign ministry had threatened a "pre-emptive nuclear attack" against the US 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.

An annual US-South Korea military exercise known as Foal Eagle is currently under way 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 the three main 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.