When diplomats trying to convince North Korea to disarm resume working group meetings next month, they will have to confront one of the thorniest issues of the six-party talks: uranium enrichment.

Progress in implementing the provisions of a February deal in which Pyongyang agreed to dismantle its nuclear programmes in return for energy aid has hardly been smooth so far. But the past six months could seem a breeze compared to discussions about Pyongyang’s efforts to enrich uranium.