"We had hoped to have a detailed and constructive discussion of those ideas. But it became clear that the Iranian side was not ready for this, unless we agreed to preconditions relating to enrichment and sanctions. Both these preconditions are not the way to proceed," said the EU policy chief.

But the head of the Iranian negotiations - Saeed Jalili - said they approached the Istanbul meeting with good will and wanted a logical approach to the talks, saying their demands weren't unreasonable.

He said, "We must respect international rights and I talked about the rights of people", "these are not preconditions , there are prerequisites."

No date has been set for new talks. but Ashton said it's up to Tehran to change its stance, saying their offer remains on the table. Observers say the collapse in talks is expected to lead to new calls for further international action against Iran. But a senior European official at the talks said it was important to be calm and take a wait and see approach.