North Korea's state media ramped its bellicose rhetoric up another notch Thursday, warning of a "thermonuclear war" as the UN Security Council prepared to vote on new sanctions against the isolated state.

Following Tuesday's announcement by the military that it would rip up the 1953 armistice ending the Korean War, the North Korean ruling party's official mouthpiece, Rodong Sinmun, warned of an imminent conflict.

"With the armistice nullified, it would be no surprise if a global thermonuclear war breaks out," the newspaper said.

"The war will not be confined to the Korean peninsula," it said, reiterating Pyongyang's claims to have rockets capable of striking the US mainland.

The warning came hours ahead of a UN Security Council meeting that is expected to adopt a resolution tightening the UN sanctions screws on North Korea after its widely condemned nuclear bomb test last month.

Amounting to one of the toughest sanctions regimes ever ordered by the United Nations, the resolution also threatens "further significant measures" if the North stages a new nuclear test or rocket launch.

Little is known about the precise nature of the North's February test. The yield was higher than its previous two nuclear detonations, but nowhere near the size that would indicate development of a thermonuclear device.

The North's official Korean Central News Agency (KCNA) on Thursday focused its vitriol on a large-scale joint US-South Korean military exercise scheduled to begin on Monday.

The drill amounted to an "unpardonable terrorist act" and a rehearsal for a "pre-emptive nuclear attack", the agency said, vowing that the North would respond with a "more powerful" nuclear strike of its own.

While most observers shrug off such rhetoric, there are concerns that tensions on the Korean peninsula are escalating to the point where even a minor border incident could swiftly snowball into something more serious.

South Korea's defence ministry says the North is preparing to hold its own large-scale military manoeuvres next week, involving all three wings of the armed forces.