Seoul has called Washington’s special envoy on North Korean human rights “biased” and “narrow-minded” as differences of opinion about how to deal with Pyongyang spilled over into unusually blunt language which is certain to further strain relations between the allies.

The two have taken markedly different approaches to North Korea – while President George W. Bush has labelled Kim Jong-il’s regime an “outpost of tyranny”, Seoul tries to avoid antagonising its neighbour. But the differences have never been stated quite so undiplomatically in public before.