SEOUL (AFP) - North Korea announced on Friday it was voiding non-aggression pacts with South Korea and severing a hotline with Seoul, hours after the UN Security Council adopted tough new sanctions on Pyongyang.

The announcement ramped up tensions on the Korean peninsula that have surged since the North staged a third nuclear test last month. On Thursday, the country threatened a pre-emptive nuclear strike against the United States.

Pyongyang is no stranger to 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.

The main bilateral non-aggression pact was signed in 1991, endorsing the peaceful settlement of disputes and the prevention of accidental military clashes.

"It also notifies the South side that it will immediately cut off the North-South hotline," said the statement, which was carried by the official Korean Central News Agency.

North Korea has threatened to sever the phone link - installed in 1971 - many times, and has actually done so twice before, South Korea's Unification Ministry noted.

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.

Earlier resolutions gave states the right to inspect cargo suspected to contain weapons material. Those inspections will become mandatory.

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 North Korean missile strike.

The CPRK statement condemned the UN resolution - drafted by the United States and the North's sole major ally, China - 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 repairable 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 exercise known as Foal Eagle began on March 1 and continues until April 30, involving more than 10,000 US troops along with a far greater number of South Korean personnel.

And a largely computer-simulated joint exercise called Key Resolve will be held from March 11 to March 21.

Meanwhile, South Korea's defence ministry said the North was gearing up for nationwide military manoeuvres of its own next week, involving all three wings of its armed forces.

In a separate report Friday, KCNA said North Korean leader Kim Jong Un had visited a frontline military unit involved in a deadly skirmish with the South in 2010.

During his inspection on Thursday, 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.