VIENNA - US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif will face off in Vienna Sunday with seven days left to bridge major differences and strike a historic nuclear deal.
Kerry and the foreign ministers of Britain, France and Germany will also use their presence in the Austrian capital to discuss an international push for a ceasefire in Gaza.
Fresh from brokering a breakthrough in a bitter standoff in Afghanistan between presidential rivals, Kerry will also try to mend ties with Germany's foreign minister in the wake of a major spying scandal in Berlin.
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.
Officials on both sides say that the talks, which on July 3 entered their sixth and final round, have made some progress, with Iran's chief negotiator saying Saturday that a draft accord was 60-65 per cent completed.
But one key notable issue remains: 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.
Iran wants to increase greatly its capacities to enrich uranium, saying it needs them to fuel its only functioning nuclear power plant - currently fuelled by Russia - as well as a fleet of future facilities.
But the six powers want a sharp reduction.
This, coupled with increased surveillance, would extend the so-called "breakout time" - the time Iran would ed 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 on 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."
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.
Many experts believe that one such step could be extending the deadline for a deal, but Washington has made clear it will not do so if Tehran does not give ground first.
"It will be hard to contemplate things like an extension without seeing significant progress on key issues," a second US official said Saturday.
"It is certainly late in the day in these negotiations, but it is not too late for Iran to take the steps that are necessary." But the Iranians are playing hard ball too, with Iran's chief negotiator Abbas Araqchi saying on Saturday that Tehran was prepared to walk away.
"If we see 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."