The US and Iran fired opening salvos Wednesday in a stormy Vienna as a final round of nuclear talks got under way with chances of a historic deal by a July 20 deadline on a knife edge.
The five permanent members of the UN Security Council plus Germany want Iran to scale down its nuclear activities in order to ease long-held fears that Tehran might develop atomic weapons.
Iran, subject to damaging UN and Western sanctions, insists its nuclear programme is purely peaceful and even wants to expand key parts of it.
This sixth round of talks starts officially on Thursday, but preliminary meetings -- including between Iran, the US and the EU -- took place on Wednesday afternoon, officials said.
The talks could potentially last until July 20 when an interim deal from November expires, although this could be extended by up to six months.
US Secretary of State John Kerry, in the Washington Post, said that the negotiations on what would be a fiendishly complex deal constituted "a choice for Iran's leaders".
"They can agree to the steps necessary to assure the world that their country's nuclear program will be exclusively peaceful... Or they can squander a historic opportunity", Kerry wrote.
The P5+1 powers have proposed to Iran a "series of reasonable, verifiable and easily achievable measures", he said.
"What will Iran choose? Despite many months of discussion, we don't know yet," he said.
British Foreign Secretary William Hague said that with differences "significant", a deal was "far from certain". Iran needs to be "realistic" about the steps it needs to take, he said.
But Iranian Foreign Minister Mohammad Javad Zarif, writing in French daily Le Monde, said that some among the P5+1 were suffering from "illusions" about Iran's nuclear programme.
He said that contrary to fears in the West, Iran -- even if it wanted to -- is "several years and not a few months" away from being able to build an atomic bomb.
Demands that Iran's programme be "radically curbed" rest on a "gross misrepresentation of the steps, time and dangers of a dash for the bomb", Zarif said.
"We are willing to provide assurances of the exclusively peaceful nature of our nuclear program. But we will not abandon or make a mockery of our technological advances or our scientists," he said.
In a video message Zarif said the talks represented a "unique opportunity to make history".
"We are trying to reach a deal," he added. "Not a good deal or a bad deal, but a doable and lasting deal."
Iranian nuclear negotiator Majid Takhte Ravanchi told the ISNA news agency that Iran has set out clear "red lines".
"The other side knows that these red lines cannot be crossed. If we reach a deal it will be one respecting these red lines. If not there will be no accord," he said.
One such position is thought to be on the key central issue of enrichment, the process rendering uranium suitable for power generation but also, when highly purified, for a bomb.
French Foreign Minister Laurent Fabius said last month Iran has to slash the number of centrifuge enrichment machines to several hundred from the almost 20,000 at present.
"We will not accept definitive restrictions" on our nuclear programme, Ravanchi said.
But Kelsey Davenport, Arms Control Association analyst, was upbeat, saying there is "considerable political will" for a deal since it is in the interests of both sides.
"There is a lot of time left for diplomacy and a good comprehensive nuclear agreement is within reach, despite significant gaps between the two sides on core issues," she told AFP.