Just before the Nortk Korea tests were announced, the Sunday Telegraph’s Sergey Soukhorukov reports from Pyongyang after speaking with Chinese and Russian diplomats there:
The nuclear weapon that North Korea intends to detonate in an underground test is big enough to kill up to 200,000 people were it ever to be used against a city such as Seoul or Tokyo, Russian military experts have revealed.
They say that the weapon, with the same 20-kiloton yield as the bomb dropped on Nagasaki, is about 10ft long and weighs four tons. It is too big to fit on to any missile Kim Jong Il’s regime currently possesses but if it were detonated above ground it could destroy everything within five square miles.
From the sound of it, the source for this was the Russian Defense Attaché in Pyongyang.