* U.S., EU sanctions officials attend nuclear talks withIran
* Washington, Tehran cautiously optimistic for progress
* West will demand Iran takes steps to ease weapons concerns
By Louis Charbonneau and Justyna Pawlak
GENEVA, Oct 14 (Reuters) - The United States held out theprospect of quick sanctions relief for Iran on Monday if Tehranmoves swiftly to allay concerns about its nuclear programme,although both countries said any deal would be complex and taketime.
Six world powers - the United States, Russia, China, France,Britain and Germany - are to hold talks with Iran on its nuclearprogramme in Geneva on Tuesday and Wednesday.
"No one should expect a breakthrough overnight," a seniorU.S. administration official told reporters.
However, the official said Washington was ready to offerIran rapid relief from economic sanctions it moved quickly toaddress concerns that the ultimate goal of its nuclear work wasto make bombs.
Any potential sanctions relief would be "targeted,proportional to what Iran puts on the table", the official said,speaking on condition of anonymity.
"I'm sure they will disagree about what is proportionate,"the official said. "But we are quite clear about what the menuof options are and what will match what."
On the eve of the talks, European Union foreign policy chiefCatherine Ashton, who represents the so-called "P5+1" nations innegotiations, had dinner with Iranian Foreign Minister MohammadJavad Zarif, who said Tehran would put its case on Tuesday.
"We had a good dinner," Zarif told Reuters as he returned tohis hotel after the two-hour dinner at the Iranian diplomaticresidence in Geneva.
When asked if he had given Ashton details of an Iranianproposal, he responded: "Proposal is for tomorrow." One diplomatsaid Zarif did not disclose Iran's proposal at the dinner.
In a hint that Washington is seriously considering easingsanctions, the U.S. delegation at the talks includes one of itsleading sanctions experts - Adam Szubin, director of theTreasury Department's Office of Foreign AssetsControl.
Undersecretary of State Wendy Sherman, effectively the StateDepartment's third-ranking diplomat, leads the U.S. delegation.
The European Union's top sanctions official has also joinedthe bloc's delegation at the talks.
Iran says its nuclear programme is peaceful and intended forenergy production.
Since 2006, it has rejected U.N. Security Council demandsthat it halt uranium enrichment and has continued to expand itsnuclear fuel programme, leading to increasingly harsh sanctions.
This week's meeting follows the June election of PresidentHassan Rouhani, a relative moderate who says he wants to thawIran's icy relations with the West to secure the removal ofpunitive sanctions that have hobbled its oil-based economy.
Foreign ministers from the P5+1 - including U.S. Secretaryof State John Kerry - met with Zarif on the sidelines of theU.N. General Assembly last month when they announced the planfor this week's meeting.
A day after Kerry met Zarif in New York, President BarackObama and Rouhani spoke by telephone, the highest levelU.S.-Iranian contact since Iran's Islamic revolution in 1979.Washington and Tehran have not had diplomatic relations since1980.
CHANGE IN TONE
Rouhani said in New York last month he wanted a deal withthe P5+1 within three to six months. Zarif played downexpectations that an agreement would be reached this week.
"Tomorrow is the start of a difficult and relativelytime-consuming way forward," he said on his Facebook page lateon Sunday. "I am hopeful that by Wednesday we can reachagreement on a road map to find a path towards resolution.
"But even with the goodwill of the other side, to reachagreement on details and start implementation will likelyrequire another meeting at ministerial level," he said.
The U.S. official said the Obama administration wasencouraged that Rouhani, who avoids the strident anti-Westernand anti-Israeli rhetoric of his predecessor MahmoudAhmadinejad, had a mandate to "pursue a more moderate course".
But Tehran must be put to the test, the official said.
"That is what we will be doing over the coming days," theofficial said, adding that "no one is naive about the challengeswe face about pursuing the diplomatic path."
"We need to see concrete verifiable actions," the officialsaid. "We go into these meetings clear-eyed that we have very,very, very difficult work to do.
"We are going to make judgements based on actions of theIranian government, not simply its words, although we appreciatethe change in its tone," the official said.
Backing up those words, 10 Democratic and Republican U.S.senators said on Monday they were open to suspending theimplementation of new U.S. sanctions, but only if Tehran tooksignificant steps to slow its nuclear programme.
The U.S. official said Washington had three prioritiesregarding Iranian assurances: Tehran must take steps on theproduction of nuclear and related material, ensure transparencyof the nuclear programme and take steps regarding its stockpileof nuclear material.
In the past, the six powers have demanded, among otherthings, that Iran halt uranium enrichment, particularly to20-percent fissile purity, move stockpiles of enriched uraniumout of the country and close down the Fordow enrichment plant,buried inside a mountain south of Tehran.
On Sunday, Iran rejected the demand that it send enricheduranium abroad but signalled flexibility on other issues.
Israel, widely assumed to be the Middle East's only statewith atomic weapons, has warned the West not to ease sanctionsbefore Tehran has addressed fears about its nuclear ambitions.
An Israeli official said on Saturday that Netanyahu hadphoned British Prime Minister David Cameron and French PresidentFrancois Hollande to tell them sanctions were close to achievingtheir goal.
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