* No reply yet from six world powers at negotiations
* Iran says proposal "serious", "logical", no details yet
* As sanctions bite, Iran says it's eager for swift deal
* Israel urges powers not to relax sanctions prematurely
* Talks seen as biggest chance in years to resolve stand-off
* Atmosphere brighter since moderate Iran president elected
By Louis Charbonneau and Justyna Pawlak
GENEVA, Oct 15 (Reuters) - A senior Iranian negotiator cautioned it was too early to say whether the Islamic Republic and six world powers made progress on Tuesday towards resolving the decade-old standoff over Tehran's disputed nuclear ambitions.
Deputy Foreign Minister Abbas Araqchi had told reporters earlier that Iran - now with a moderate president committed to easing its international isolation - had presented a proposal capable of achieving a breakthrough in the deadlock.
But Araqchi was more circumspect when he spoke to Reuters after the first day of a planned Oct. 15-16 meeting in Geneva between Iran and the United States, Russia, China, France, Britain and Germany.
"It's too soon to judge," he said when asked whether the two sides were any closer to resolving a stalemate that has heightened the risk of a new Middle East war.
The West suspects Iran is trying to develop the means to make nuclear weapons behind the screen of a declared civilian atomic energy programme. Tehran denies this but its refusal to limit activity applicable to producing atomic bombs, or to permit unfettered U.N. inspections, has drawn severe sanctions.
Iran began negotiations in earnest with the six powers two months after President Hassan Rouhani took office promising conciliation over confrontation in relations with the world.
After years of ideological defiance, Iran showed up in Geneva keen for a deal to win relief from sanctions that have crippled its economy, slashed 60 percent of its daily oil export revenue and wrought a steep devaluation of its rial currency.
Details of the Iranian proposal - unveiled as a nearly hour-long PowerPoint presentation - were not immediately available.
Western diplomats have cautioned in the past that Tehran has refused to offer sufficient nuclear concessions to warrant a deal. But both sides signalled that the atmosphere, at least, in Tuesday's initial session was positive.
In a possible sign of the Islamic Republic's determination to meaningfully address specifics of the powers' concerns, the talks in Geneva were conducted in English for the first time.
A spokesman for the European Union foreign policy chief, Catherine Ashton, who oversees diplomacy with Iran on behalf of the powers, described the Iranian presentation on Tuesday morning as "very useful" but did not elaborate.
The spokesman, Michael Mann, later tweeted that "for the first time, very detailed technical discussions continued this afternoon between (the six powers) and Iran on the Iranian nuclear programme".
Iranian Foreign Minister Mohammad Javad Zarif, its chief nuclear negotiator, and Ashton were to meet later on Tuesday and the full negotiations would resume on Wednesday, he said.
A State Department spokeswoman said Washington would welcome a bilateral meeting with Iran on the sidelines, suggesting U.S. officials felt a stripped down, separate session with the Iranians could be key to bridging differences.
Undersecretary of State Wendy Sherman, effectively the State Department's third-ranking diplomat, leads the U.S. delegation. If bilateral talks took place, it was not immediately clear who Sherman might face - Zarif or Araqchi.
Washington and Tehran have been locked in mutual enmity since diplomatic ties were broken in 1980 - an estrangement that has posed a significant obstacle to any nuclear deal - but the two revived high-level contact at the United Nations last month.
On Monday, U.S. officials held out the prospect of quick sanctions relief if Tehran acted swiftly to allay concerns about its nuclear programme, although both countries said any deal would be complex and take time.
At the core of the dispute are Iranian efforts to enrich uranium to 20 percent fissile purity, a technological advance that brings it close to producing weapons-grade fuel.
Iran has previously spurned Western demands that it abandon such work as an initial step to build confidence in return for modest sanctions relief, and repeatedly called for the most painful limits on trade, such in the oil sector, to be lifted.
Western diplomats have said their demands related to 20-percent uranium must be addressed before further progress is made. But some diplomats acknowledged ahead of the Geneva talks that their initial offer to Iran might be changed substantially depending on what concessions Iran offered.
In comments made to Iranian media, Araqchi said that any final deal should eliminate sanctions on Iran and enshrine its "right" to refine uranium, according to the ISNA news agency.
A U.S. administration official said any potential cutback of sanctions would be "targeted, proportional to what Iran puts on the table. No one should expect a breakthrough overnight".
Israel, Iran's arch-enemy and widely assumed to harbour the Middle East's only nuclear arsenal, has lobbied Western powers not to dilute sanctions before Iran has tackled core concerns - enrichment and lack of transparency - about its nuclear goals.
Israel's air force, in an effort to telegraph its ability to launch long-range strikes, carried out major drills over the Mediterranean last week and on Monday.
The first drill, which involved aerial refueling for fighter jets, was published by the Israeli military and the latest was disclosed to Reuters by a security source on Tuesday.
Israel's security cabinet meanwhile urged the powers to demand a complete rollback of Iran's enrichment programme - something some Western diplomats say may no longer be realistic given its size and identification by the Iranian leadership with national pride and sovereignty.
Since 2006, Iran has rebuffed U.N. Security Council demands that it shelve enrichment and has continued to expand its nuclear fuel programme, triggering ever stiffer sanctions.
Hopes of a negotiated settlement of the dispute rose last month when President Barack Obama and Rouhani spoke by telephone, the loftiest U.S.-Iranian contact since Iran's Islamic Revolution in 1979.
Zarif, the Iranian foreign minister, attended the morning session of talks but not the afternoon round. He has been suffering from s back ailment and told reporters on returning to his hotel: "I'm really in pain."