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 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 on Saturday that a draft accord was 60-65 percent 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 be 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.

(Agencies)

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