* Nascent compromise on Iran's stated "right to enrich" uranium
* Fate of Iran's Arak heavy-water reactor remains key dispute
* France's Fabius says in Geneva to reach "solid deal"
* China, British, French, German, Russian foreign ministers arrive
* Iranian negotiator sees deal close, some differences remain
* Powers aim to ease risk of Iran developing nuclear weapon
(Adds Russian, German, Iranian comment on progress, detail)
GENEVA, Nov 23 (Reuters) - U.S. Secretary of State John Kerry arrived in Geneva on Saturday to join talks on Iran's contested nuclear programme with Tehran and six world powers appearing on the verge of a breakthrough to defuse the decade-old standoff.
The Chinese, Russian, French, British and German foreign ministers - Wang Yi, Sergei Lavrov, Laurent Fabius, William Hague and Guido Westerwelle - also prepared to take part in the push to seal an interim deal under which Iran would curb its nuclear work in exchange for limited relief from sanctions.
Diplomats said a formidable sticking point in the intense negotiations, which began on Wednesday, may have been overcome with compromise language that does not explicitly recognise Iran's claim to a "right to enrich" uranium but acknowledges all countries' right to their own civilian nuclear energy. 1/8ID:
But Russian Deputy Foreign Minister Sergei Ryabkov said Iran's demand to continue construction of a heavy-water reactor near Arak that could, when operational, yield bomb-grade plutonium remained one of the main outstanding issues.
Ryabkov said a breakthrough was closer now than at the Nov. 7-9 round of Geneva talks but, he told Russia's Itar-Tass news agency, "unfortunately I can't say that there is a certainty of reaching that breakthrough."
"It's not a done deal. There's a realistic chance but there's a lot of work to do," Germany's Westerwelle told reporters.
The powers' goal is to cap Iran's nuclear energy programme, which has a history of evading U.N. inspections and investigations, to remove any risk of Tehran of refining uranium to a level suitable for bombs rather than electricity.
"We are close to a deal but still differences over two-three issues remain," said Iranian Deputy Foreign Minister Abbar Araqchi, a senior negotiator.
A senior European diplomat had told reporters earlier that foreign ministers of the six states would come to Geneva only if there was a consummated agreement to sign.
An interim accord on confidence-building steps would be designed to start a cautious process of detente with Iran after decades of estrangement, and banish the spectre of a devastating Middle East war over its nuclear quest.
The draft deal would have Iran suspend some sensitive nuclear activities, above all medium-level enrichment, in exchange for the release of some of many billions of dollars in Iranian funds frozen in foreign bank accounts, and renewed trade in precious metals, petrochemicals and aircraft parts.
The United States might also agree to relax pressure on other countries not to buy Iranian oil. Tehran has made clear it wants a more significant dilution of the sanctions blocking its oil exports and use of the international banking system.
France's Fabius, who objected to what he felt was a one-side offer to Iran floated at the November 7-9 negotiating round, appeared guarded on arrival in Geneva early on Saturday.
"I hope we can reach a deal, but a solid deal. I am here to work on that," he said. France has consistently taken a tough line over Iran's nuclear programme, helping Paris cultivate closer ties with Tehran's adversaries in Israel and the Gulf.
TALKS AT "FINAL MOMENT"?
Kerry left for Geneva "with the goal of continuing to help narrow the differences and move closer to an agreement," State Department spokeswoman Jen Psaki said.
Echoing optimism that a deal was close, China's state-run Xinhua news agency quoted a foreign ministry spokesman as saying the talks "have reached the final moment".
The United States and other Western powers say there is no such thing under the nuclear Non-Proliferation Treaty as a "right to enrich", but Iran has viewed this as a matter of national sovereignty and crucial to any deal.
Diplomats said new, compromise wording on the table did not explicitly recognise a right to produce nuclear fuel by any country. "If you speak about the right to a peaceful nuclear programme, that's open to interpretation," a diplomat said.
Iran also wants relief from sanctions that have severely damaged its oil-dependent economy in return for any nuclear concessions it makes that could allay the West's suspicions about its stockpiling of enriched uranium.
For the powers, an interim deal would mandate a halt to Iran's enrichment of uranium to a purity of 20 percent - a major technical step towards the bomb threshold, more sweeping U.N. nuclear inspections in Iran and an Arak reactor shutdown.
If a preliminary agreement is reached, it would run for six months that would provide time for the powers and Tehran to hammer out a broader, longer-term settlement.
Diplomacy on Tehran's nuclear aspirations has revived remarkably since the election of Hassan Rouhani, a relative moderate, as president in June on promises of winning sanctions relief and diminishing Iran's international isolation.
The sides have struggled to wrap up a deal, however, bogged down in politically vexed details and hampered by long-standing mutual mistrust.
The OPEC producer rejects suspicions it is covertly trying to develop the means to produce nuclear weapons, saying it is stockpiling nuclear material for future atomic power plants.
Israel pursued its public campaign against the offer of respite from sanctions for Iran, voicing its conviction that all this would achieve would be more time for Iran to master nuclear technology and amass potential bomb fuel.
Israeli Prime Minister Benjamin Netanyahu told local media in Moscow that Iran was essentially given an "unbelievable Christmas present - the capacity to maintain this breakout capability for practically no concessions at all."
(Additional reporting by Arshad Mohammed, Louis Charbonneau, Justyna Pawlak, Fredrik Dahl and Katya Golubkova in Geneva, Marcus George in Dubai, John Ruwitch in Shanghai, Steve Gutterman in Moscow, Allyn Fisher in Jerusalem, Hortense de Roffignac in Paris, Lesley Wroughton in Washington; Editing by Mark Heinrich)