Britain has joined calls to North Korea to calm escalating rhetoric after the reclusive communist state declared a "state of war" against its southern neighbour.
The threat is the latest in a series since the United Nations imposed new sanctions in response to the country's latest nuclear test last month.
Young leader Kim Jong Un threatened the US on Friday after its B2 bombers took part in an annual training exercise with South Korea.
Analysts believe the inflammatory language is aimed at pushing Washington into talks and that the regime would not risk full-blown conflict, but tensions are high.
A Foreign Office spokesman said: "We have noted today's statement from North Korea.
"We have made clear to North Korea that its long-term interests will only be served by constructive engagement with the international community. These threatening statements will only seek to isolate it further. The armistice agreement has enabled the Korean peninsula to benefit from 60 years' peace. Maintaining it is in the best interests of all."
A statement issued via Pyongyang's state news agency said that "from this time on, the North-South relations will be entering the state of war and all issues raised between the North and the South will be handled accordingly".
Any military provocations "will not be limited to a local war, but develop into an all-out war, a nuclear war", it added.
The two parts of the peninsular have technically been at war for 60 years as no formal peace treaty was ever signed at the end of the 1950-53 war.
Its regular threats - including of a nuclear strike - against the South and American military bases are seen as an effort to secure a change of attitude from Seoul, talks with the US that could lead to extra aid and to strengthen Kim Jong Un's military credentials.