World leaders were swift to denounce the underground explosion that sent shockwaves through the region and beyond. But they struggled to find a swift formula that would have any chance of forcing North Korea’s nuclear genie back into the bottle.

The United States called for the right to blockade the Stalinist state, with foreign powers allowed to intercept all cargoes arriving and leaving by land or sea.

The UN Security Council put on a united front and demanded that North Korea return to the negotiating table. Britain called for tough and binding sanctions.

The five UN members with veto powers — the US, France, Britain, Russia and China — plus Japan met last night to discuss the US stance. The American proposal calls for: “Appropriate provisions authorising international inspection of all cargo to or from North Korea to limit proliferation of nuclear, chemical and biological weapons, as well as their means of delivery and components.”

Japan late last night asked for a draconian ban on all North Korean ships and aircraft entering other UN member states as well as an embargo on North Korean exports.

Diplomats said that Japan was working closely with the US and that its more stringent demands reflected tactical coordination with Washington.

The diplomatic response, and the likely wrangling to follow before it is translated into action, contrasted with the might of the blast, which Russia estimated at between 5 and 15 kilotonnes. However, France estimated the blast at about 1 kilotonne.

The test served only to emphasise the enormity of North Korea’s carefully calculated decision to become the ninth member of the world’s nuclear club. The Far East now has its first new nuclear power for 40 years, giving Japan an avowedly hostile nuclear neighbour. Other states with nuclear ambitions will look closely at how the world responds.

The timing of the test was deeply symbolic. Not only did North Korea make the nuclear grade before Iran, it appeared designed to overshadow the appointment of the new UN Secretary-General, Ban Ki Moon, the former South Korean Foreign Minister. It also sent a message of defiance to the warming of relations between China and Japan.

North Korea’s new UN Ambassador chided the world for being so hard on his nation. Pak Gil Yon said: “It is better for the Security Council of the United Nations to congratulate the Democratic People’s Republic of Korea scientists and researchers.”

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