A three-metre-high metal fence topped with razor wire in a North Korean port marks the front line of the United Nations' ban on coal exports by Pyongyang.

A mountain of North Korean coal -- which would once have been bound for China -- is piled up on one side of the barrier in Rajin harbour, stranded by the interdiction.

On the very next dock, around two million tonnes of Russian coal has come in by train and been shipped on to China this year by Russian port operator RasonConTrans.

Its activities are specifically excluded from the UN Security Council's sanctions resolutions -– but attempts have been made to use it as a way to bypass the restrictions.