Britain and Germany's foreign ministers will join US Secretary of State John Kerry in Vienna this weekend to seek to bridge what Washington called "significant gaps" in nuclear talks with Iran, London and Berlin said Friday.
The negotiations between Iran and the five permanent members of the UN Security Council plus Germany, which last week entered the final straight, are aimed at forging a lasting deal with Iran meant to ease fears that the Islamic state might develop nuclear weapons.
The deadline to get a deal is July 20, when an interim deal struck by foreign ministers in Geneva last November expires, although this can be extended if both sides agree.
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," spokeswoman Marie Harf said in an emailed statement late Thursday.
He "will see if progress can be made on the issues where significant gaps remain and assess Iran's willingness to make a set of critical choices at the negotiating table" and then "make recommendations" to President Barack Obama on the next steps.
Kerry will also hold talks in Vienna with Iranian Foreign Minister Mohammad Javad Zarif, Harf said.
London on Friday confirmed British Foreign Secretary William Hague's attendance, as did Germany for Frank-Walter Steinmeier. In Paris a diplomatic source told AFP that Foreign Minister Laurent Fabius would also be there.
But Russian Foreign Minister Sergei Lavrov will not be present, with Moscow represented by Vladimir Voronkov, Russia's ambassador to international organisations in the Austrian capital, ITAR-TASS cited Voronkov as saying.
Chinese foreign ministry spokesman Qin Gang said he had "no specific information about which Chinese official will go, but we will send an official of some level to the meeting".
On Thursday the lead negotiator, EU foreign policy chief Catherine Ashton, invited all foreign ministers from the six powers to Vienna to "take stock of where we are", her spokesman said.
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- Breakouts and fractions -
The negotiations have, as expected, proved tough going.
France has described them as "difficult" and said no major issues had been resolved, with Foreign Minister Laurent Fabius hinting Tuesday at differences between Russia and Western countries in the talks.
French foreign ministry Romain Nadal on Friday sought to play down the divisions, saying the six powers are "united on one goal: obtaining a credible, complete and sustainable agreement, which provides all the necessary guarantees that Iran will not seek to acquire nuclear weapons".
Russia said Thursday there were "clear signs of progress" in talks.
The main sticking point 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.
The six powers want a sharp reduction, however, with a senior US official saying last week that Iran's activities in this area should be a "fraction" of what they are now.
This, coupled with other measures, would extend the so-called "breakout time" -- the time Iran would need to make enough highly-enriched uranium for a nuclear weapon, should it choose to do so.
Iran says it wants to enrich uranium to fuel planned nuclear power plants around Iran, but these facilities are years, if not decades, away from being in operation, the West says.
Kelsey Davenport, analyst at the Arms Control Association, told AFP that the attendance of foreign ministers was a "positive sign" and that their presence could "spur movement on some of the largest remaining hurdles".
Their presence would also facilitate discussing extending the deadline, although for now this remains "premature," Davenport said, with "a lot of time left on the clock for diplomacy before July 20."