For a Hong Kong company managing a small fleet of cargo ships, Union Link International hoists an exotic array of flags, from China and Tanzania to the Pacific island nations of Niue and Palau.

But one stands out: a five-pointed red star on broad horizontal stripes of red and dark blue. It is North Korea’s national banner and the vessel that flies it, the Dolphin 26, is just one in a vast network helping Pyongyang survive the international sanctions that target its rapidly advancing weapons programme.

The UN Security Council on Monday passed its toughest measures yet against North Korea and reiterated an agreement to identify and inspect vessels suspected of transporting proscribed goods from the country. But the opacity of the web of ships, people and businesses perpetuating such trade flows suggests the international community faces a huge challenge if its clampdown is to succeed.