US and UN have been speculating about North Korea's nuclear capabilities for quite some time, but recently they have accepted that North Korea has emerged as a Nuclear Power.
Following underground Nuclear tests in 2006, concerns rose about North Korea's intention and the extent of their success in the Nuclear race, but the US shrugged off the anxieties stating that such bombs couldn't be a threat since they (North Korea) would not be able to fit them into missiles as warheads.
However, learned officials ranging from the US Secretary of State to the head of the International Atomic Energy Agency have acknowledged that North Korea has the ability to strike South Korea and the neighbouring regions- upto Japan with Nuclear warheads.
They don't have the long range missiles to be a threat anywhere else, but nevertheless their weapons could be devastating on South Korean and Japanese cities.
It must be noted that Japan is the only country to have been the victim of nuclear attacks- only 2 in history, bombed by the US on Aug 6th 1945 and Aug 9th 1945 on Hiroshima and Nagasaki respectively.
Japan is not a nuclear powered state and hence with the newly acquired nuclear arsenal, North Korea could find Japan a soft target in case of War.
South Korea expressed concerns over naming North Korea as a nuclear state since it would give it more leverage in case of negotiations.