* First talks since election of moderate Iranian president
* Powers looking for substance in Rouhani's emollient stance
* Iran foreign minister sees tough "time-consuming" process
* But sees scope for "roadmap" towards defusing stand-off
* Talks start Tuesday, US sanctions expert Sherman on hand
DUBAI, Oct 14 (Reuters) - Foreign Minister and chief negotiator Mohammad Javad Zarif voiced hope Iran and world powers can agree at talks this week on a roadmap towards defusing the stand-off over Tehran's nuclear activity, but warned the process would be complex.
The negotiations about Iran's nuclear programme, to start in Geneva on Tuesday, will be the first since the June election of President Hassan Rouhani, a relative moderate who wants to thaw Iran's icy relations with the West to secure the removal of punitive sanctions that have hobbled its oil-based economy.
Western nations believe Iran's uranium enrichment programme is covertly meant to achieve a nuclear arms capability. Tehran denies this, saying it wants only to master nuclear technology to generate electricity and carry out medical research.
"Tomorrow is the start of a difficult and relatively time-consuming way forward. I am hopeful that by Wednesday we can reach agreement on a road map to find a path towards resolution," Zarif said in a message posted on his Facebook account late on Sunday.
"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."
Rouhani's election in June to succeed conservative hardliner Mahmoud Ahmadinejad has raised hopes of a negotiated solution to a decade-old dispute over the programme that could otherwise kindle a new war in the tinderbox Middle East.
"We will see if there is a way to transform this new attitude into gestures, but up to now, beyond the new attitude, there has been a total absence of anything that takes us forward on the fundamentals," a Western diplomat said.
"We're expecting that things are more open, but at the same time more complicated as we'll have to study what they are offering," said the diplomat, who declined to be named.
The diplomat added that if the Islamic Republic failed to put any serious new proposal on the table "after all this talk, then they have a serious problem".
Zarif's deputy on Sunday rebuffed the West's demand that Iran send sensitive nuclear material abroad but signalled flexibility on other aspects of its atomic activities, including the degree of uranium enrichment, that worry global powers.
IRAN WANTS BETTER OFFER ON SANCTIONS RELIEF
In sporadic talks since early 2012, the world powers have demanded Iran take initial confidence-building steps including suspending 20 percent enrichment, relinquishing some of its existing refined uranium stockpile and closing the underground Fordow plant where most higher-grade enrichment is carried out.
In return, they have offered to rescind sanctions on Iranian trade in gold, precious metals and petrochemicals. Tehran has dismissed that offer, calling for the removal of oil and banking restrictions most damaging to its economy.
However, in a hint that Washington may be devoting greater thought to how it might relax sanctions, its Geneva delegation will include one of the U.S. government's leading sanctions experts, U.S. Undersecretary of State Wendy Sherman.
Tehran says it needs uranium refined to 20 percent fissile purity to produce isotopes for medical care. But the powers are wary that 20 percent is only a short technical step away from bomb-grade uranium and such a stockpile could give Iran a quick route to weaponisation without stricter limits on its activity.
Iran also wants the six powers - the United States, Russia, China, Britain, France and Germany - to recognise what it regards as its sovereign "right" to enrich uranium.
"We continue to believe that while there is a significant chance of a deal by the end of the second quarter of 2014, an agreement on balance remains improbable," Middle East analyst Cliff Kupchan of risk consultancy Eurasia group said.
"Iran will likely offer a new proposal in which it sets out a roadmap, possibly including concessions on medium-enriched uranium in return for sanctions relief," he wrote in a commentary. "The U.S. will agree to study the proposal but probably insist on more severe near-term constraints on Iran's nuclear programme."
Israel, which has threatened pre-emptive military action against its arch-enemy Iran if it deems diplomacy a dead end, demands the total removal of Tehran's enriched uranium reserves along with a dismantling of its enrichment plants.
Western officials have acknowledged this maximal demand - incorporated in U.N. Security Council sanctions resolutions since 2006 - may no longer be realistic given the meteoric growth of Iran's enrichment infrastructure, and the way it has made nuclear energy and know-how synonymous with national pride.
But they say Iran's enrichment capacity must be kept in check to make it harder for Tehran to weaponise enrichment, should it decide to do so, without being detected in time.