North Korea has warned Seoul and Washington that any provocation could swiftly escalate into an all out nuclear conflict, after declaring it's in a 'state of war' with South Korea.

The White House says it is taking the announcement seriously, describing the latest statement as 'unconstructive'.

It is the latest in a string of aggresive warnings from North Korea, which is fueling international concern that the situation may spiral out of control.

But South Korea has largely dismissed it as an old threat as its Defence Ministry claims that nothing has changed.