North Korea responded to new UN sanctions on Friday with fresh threats of nuclear war, the scrapping of peace pacts with South Korea and the severing of a hotline with Seoul.
The latest measures announced by Pyongyang ramped up tensions on the Korean peninsula that have surged since the North staged a third nuclear test last month.
On Thursday, the country had threatened a pre-emptive nuclear strike against the United States and South Korea.
Pyongyang is known for its bellicose rhetoric, but the tone has reached a frenzied pitch in recent days, fuelling concerns that it might trigger a border incident, with both North and South planning major military exercises next week.
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 a statement.
A non-aggression pact signed in 1991 endorsed the peaceful settlement of disputes and the prevention of accidental military clashes.
The CPRK said the pact 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 added new names to the UN sanctions blacklist and 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 and to defuse tensions.
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.
The CPRK statement Friday 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 on the Korean Peninsula where a nuclear war may break out right now," it said.
The statement warned that the North Korean military would respond "mercilessly" to any intrusion -- "even an inch" -- into its land, sea or air space.
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 also believed to be gearing up for nationwide military manoeuvres of its own next week, involving all three wings of its armed forces.
While most observers dismiss the North's nuclear war threats as bluster, there are fears about the volatile mix of hair-trigger tension and military exercises.
"There's always that risk of a miscalculation and rapid escalation," said 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 attacks in all frontline areas in case of any provocation, KCNA said.