North Korea's leader has placed his army on readiness to "annihilate the enemy" as the regime officially renounced all non-aggression pacts with its southern neighbour.
Kim Jong-un escalated his inflammatory response to a new round of United Nations sanctions by ordering troops on the border with South Korea to prepare for war.
But China, North Korea's only powerful ally, publicly urged "calm and restraint".
Mr Kim chose a highly sensitive location for his address to North Korean troops, visiting military positions facing the South Korean island of Yeonpyeong, where a North Korean bombardment killed four people and wounded 19 in 2010.
Mr Kim "stressed the need for the soldiers to keep themselves fully ready to go into action to annihilate the enemy".
His open talk of a new Korean war came after his regime threatened America with a "pre-emptive" nuclear strike. The belligerence came in response to the unanimous vote in the UN Security Council on Thursday to impose further economic sanctions.
These measures followed North Korea's third test of a nuclear bomb last month.
In addition, Mr Kim has ended the hotline between North and South, which was designed to defuse crises, and renounced various non-aggression pacts between the two countries.
Despite all the posturing, experts believe that North Korea is highly unlikely to start a war. South Korea benefits from a US security guarantee, meaning that Mr Kim would almost certainly lose any conflict.
The biggest danger was that fighting could break out by accident as both sides raised their alert level.
China's patience with North Korea appears to be wearing thin.
Zhang Liangui, a Korea expert at the Central Party School, said the fact that North Korea had chosen to press ahead regardless of Chinese objections and international pressure "shows their internal policy, and their policy to the outside world, is hardening". (© Daily Telegraph, London)