Iran's chief negotiator in nuclear talks in Vienna warned Saturday that Tehran is ready to walk away if "excessive" Western demands cause a failure, a day before foreign ministers try to inject momentum.
Eight days before a deadline for a deal, Abbas Araqchi said however that he hoped that the attendance from Sunday of foreign ministers including US Secretary of State John Kerry would help overcome "deep differences" that remain.
"If we see that the excessive demands (of Western powers) persisting and that a deal is impossible, this is not a drama, we will continue with our nuclear programme," Araqchi said.
"The presence of ministers will have a positive influence," he told Iran state television from the Austrian capital. "There are questions that ministers need to take decisions on."
Iran's talks with the five permanent members of the UN Security Council plus Germany are aimed at a grand bargain reducing in scope Iran's nuclear activities in return for sanctions relief.
Such a deal is meant to quash for good concerns about the Islamic republic getting the bomb after more than a decade of failed diplomacy, threats of war and atomic expansion by Iran.
Iran denies wanting nuclear weapons. The deadline for an accord is July 20, when an interim accord struck by foreign ministers expires, although this can be put back if both sides agree.
Kerry was expected late Saturday or early Sunday in Vienna where he will be joined by his British, French and German counterparts William Hague, Laurent Fabius and Frank-Walter Steinmeier.
Hague said on Saturday that the Western ministers would also discuss how to achieve a ceasefire in Gaza. Kerry and Steinmeier were also to talk about a US-German spat over spying.
Skipping the meeting however is Russian Foreign Minister Sergei Lavrov, and it remains unclear who will represent China.
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- Critical choices -
Kerry "will gauge the extent of Iran's willingness to commit to credible and verifiable steps that would back up its public statements about the peaceful nature of its nuclear programme," the State Department said.
He will "assess Iran's willingness to make a set of critical choices at the negotiating table" and then "make recommendations" to US President Barack Obama on the next steps.
Some progress has been made in drafting the actual deal, with Araqchi saying that both sides saw eye to eye on "60-65 percent" of issues.
But he added that there were still "deep differences" on the "fundamental issues".
The main problem area is uranium enrichment, a process which can produce nuclear fuel -- Iran's stated aim -- but also in highly purified form the core of an atomic weapon.
On Tuesday Ayatollah Ali Khamenei, Iran's supreme leader, gave a speech indicating that Tehran intends to greatly increase its enrichment capacities to fuel a future fleet of nuclear power stations.
The six powers want a sharp reduction, however.
This, coupled with increased surveillance, would extend the so-called "breakout time" -- the time Iran would need to make enough highly-enriched uranium for a nuclear weapon, if it choose to do so.
"We have made some progress but on some key issues, Iran has not moved from their ... unworkable and inadequate positions", a senior US official said Saturday.
"There is no question that we have heard about Iran's aspirations for its nuclear programme in very specific terms and very specific numbers, and that remains far from a significant reduction in their current programme."