The decision came during talks between the five permanent members of the UN Security Council and Germany.

The six nations - China, France, Russia, the UK and US - had told Iran in June there would be no further economic sanctions if it suspended uranium enrichment as an initial step towards starting talks on a longer-term deal.

But the US state department said a letter earlier this week in which Iran called for "mutual clarification" failed to give a response.

The White House described the letter as a "stalling tactic", while France said it regretted that Iran had "once again chosen not to answer clearly". The UK also expressed disappointment at Iran's response.

But Russia's UN Ambassador Vitaly Churkin said that despite the ambiguous response, there was still hope for ongoing dialogue. He said that although a fourth round of sanctions had been raised in the discussion, "there have been no firm agreements or understandings or any kind of concerted work in this regard".