* U.S., EU sanctions officials attend nuclear talks with Iran
* 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 the prospect of quick sanctions relief for Iran on Monday if Tehran moves swiftly to allay concerns about its nuclear programme, although both countries said any deal would be complex and take time.
Six world powers - the United States, Russia, China, France, Britain and Germany - are to hold talks with Iran on its nuclear programme in Geneva on Tuesday and Wednesday.
"No one should expect a breakthrough overnight," a senior U.S. administration official told reporters.
However, the official said Washington was ready to offer Iran rapid relief from economic sanctions it moved quickly to address concerns that the ultimate goal of its nuclear work was to 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 menu of options are and what will match what."
On the eve of the talks, European Union foreign policy chief Catherine Ashton, who represents the so-called "P5+1" nations in negotiations, had dinner with Iranian Foreign Minister Mohammad Javad Zarif, who said Tehran would put its case on Tuesday.
"We had a good dinner," Zarif told Reuters as he returned to his hotel after the two-hour dinner at the Iranian diplomatic residence in Geneva.
When asked if he had given Ashton details of an Iranian proposal, he responded: "Proposal is for tomorrow." One diplomat said Zarif did not disclose Iran's proposal at the dinner.
In a hint that Washington is seriously considering easing sanctions, the U.S. delegation at the talks includes one of its leading sanctions experts - Adam Szubin, director of the Treasury Department's Office of Foreign Assets Control.
Undersecretary of State Wendy Sherman, effectively the State Department's third-ranking diplomat, leads the U.S. delegation.
The European Union's top sanctions official has also joined the bloc's delegation at the talks.
Iran says its nuclear programme is peaceful and intended for energy production.
Since 2006, it has rejected U.N. Security Council demands that it halt uranium enrichment and has continued to expand its nuclear fuel programme, leading to increasingly harsh sanctions.
This week's meeting follows the June election of President Hassan Rouhani, a relative moderate who says he wants to thaw Iran's icy relations with the West to secure the removal of punitive sanctions that have hobbled its oil-based economy.
Foreign ministers from the P5+1 - including U.S. Secretary of State John Kerry - met with Zarif on the sidelines of the U.N. General Assembly last month when they announced the plan for this week's meeting.
A day after Kerry met Zarif in New York, President Barack Obama and Rouhani spoke by telephone, the highest level U.S.-Iranian contact since Iran's Islamic revolution in 1979. Washington and Tehran have not had diplomatic relations since 1980.
CHANGE IN TONE
Rouhani said in New York last month he wanted a deal with the P5+1 within three to six months. Zarif played down expectations that an agreement would be reached this week.
"Tomorrow is the start of a difficult and relatively time-consuming way forward," he said on his Facebook page late on Sunday. "I am hopeful that by Wednesday we can reach agreement on a road map to find a path towards resolution.
"But even with the goodwill of the other side, to reach agreement on details and start implementation will likely require another meeting at ministerial level," he said.
The U.S. official said the Obama administration was encouraged that Rouhani, who avoids the strident anti-Western and anti-Israeli rhetoric of his predecessor Mahmoud Ahmadinejad, 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," the official said, adding that "no one is naive about the challenges we face about pursuing the diplomatic path."
"We need to see concrete verifiable actions," the official said. "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 the Iranian government, not simply its words, although we appreciate the 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 the implementation of new U.S. sanctions, but only if Tehran took significant steps to slow its nuclear programme.
The U.S. official said Washington had three priorities regarding Iranian assurances: Tehran must take steps on the production of nuclear and related material, ensure transparency of the nuclear programme and take steps regarding its stockpile of nuclear material.
In the past, the six powers have demanded, among other things, that Iran halt uranium enrichment, particularly to 20-percent fissile purity, move stockpiles of enriched uranium out 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 enriched uranium abroad but signalled flexibility on other issues.
Israel, widely assumed to be the Middle East's only state with atomic weapons, has warned the West not to ease sanctions before Tehran has addressed fears about its nuclear ambitions.
An Israeli official said on Saturday that Netanyahu had phoned British Prime Minister David Cameron and French President Francois Hollande to tell them sanctions were close to achieving their goal.