South Korea and the US have reached agreement on nuclear co-operation after five years of talks. But they failed to agree on Seoul’s demand to be allowed to enrich uranium and reprocess spent fuel.

The renegotiation of an expiring 1973 deal has become one of the thorniest issues between Washington and Seoul, a regional ally that has chafed at the US’s refusal to allow it to follow others such as Tokyo in developing nuclear technology.

US ambassador Mark Lippert hailed the deal as “sophisticated and dynamic” after a signing on Wednesday. But while the agreement promised extensive consultation on South Korean enrichment and reprocessing, it left open the question of whether Seoul would ultimately be permitted to achieve these goals — seen as a sovereign right by some in South Korea but sitting uneasily with the Obama administration’s efforts to tackle the proliferation of nuclear weapons technology.