A senior US official is to meet EU negotiators involved in talks with Iran over its nuclear programme. The meeting in London follows reports that Iran has begun to process a new batch of uranium at its Isfahan plant.
Washington called the move "unwelcome" - a week before the UN's atomic watchdog meets to decide whether to refer Iran to the Security Council. The US suspects Tehran of wanting to develop nuclear weapons - something Tehran has repeatedly denied.
Nicholas Burns, the undersecretary of state for political affairs, will hold talks with officials from France, UK and Germany. US state department spokesman Adam Ereli said Tehran's decision to resume processing was "one we view with concern".
"It is the latest in a series of moves by Iran that, frankly, go against what they themselves have committed themselves to and what the international community has asked of them," he said. Mr Burns and his counterparts will "consider again ... how together we can all act to accomplish our common goal," Mr Ereli said.
The EU Three - Britain, France and Germany - have been in negotiation with Tehran over concerns about its nuclear programme. The UN's nuclear watchdog, the International Atomic Energy Agency, is finalising a report on Iran's nuclear programme ahead of a meeting next Thursday in Vienna.
The agency's 35-nation board of governors will consider whether to refer Iran to the Security Council for possible sanctions. I.L.