But they report little progress on the main dispute, with Iran resisting US-led attempts to place strict constraints on its uranium enrichment programme.
Enriched uranium can be used for reactor fuel or to arm nuclear weapons, depending on the level of enrichment. Iran says it does not want such arms and is pushing to keep its present enrichment capacity.
Washington and its allies seek deep cuts in the nearly 20,000 centrifuges Iran has operating or on standby, and other restrictions.