* UN atom agency to again press for visit at Friday talks
* Parchin may have been "cleansed" of nuclear evidence -diplomats
* Iran denies illicit nuclear weapons research done there
By Ritsuko Ando and Fredrik Dahl
HELSINKI/VIENNA, Aug 22 (Reuters) - The U.N. nuclear watchdog chief played down chances of a breakthrough when talks with Iran resume on Friday but said the agency would pursue access to a military site that diplomats say may have been cleansed of evidence of illicit nuclear activity.
Visiting the Parchin complex has become a priority for the International Atomic Energy Agency as it seeks to end what the West sees as Iranian stonewalling of an IAEA investigation into allegations that Tehran has sought to design a nuclear weapon.
"I cannot be too optimistic ... We have been making our best efforts in a constructive spirit to work out an agreement between Iran and IAEA, but so far we have not been successful in reaching agreement," agency Director General Yukiya Amano told reporters on Wednesday during a visit to Finland.
"I have no indication this will change very soon," he said.
Citing satellite images, Western diplomats say Iran has demolished some small buildings and moved earth at Parchin in an apparent attempt to purge incriminating evidence from a site where the IAEA believe tests in a steel chamber relevant to nuclear arms were carried out, possibly a decade ago.
Amano said the IAEA still wanted access, but that apparent efforts to sanitise Parchin could impede the agency's inquiry.
"Through the satellite imagery we think that Iran is moving soil, demolishing buildings, using water, removing fences, doing landscape activities. We think this would hamper our verification activities," he said, echoing previous comments.
"Nevertheless we keep on requesting Iran to give us access to the building at the site of Parchin."
Iran denies accusations that it wants to develop nuclear arms technology, saying it is after civilian atomic energy only.
But its refusal to open up its nuclear work to unfettered IAEA inspections that could pin down whether it is purely peaceful or not has led to tougher international sanctions against Iran and heightened speculation that Israel, Tehran's arch-enemy, might bomb its nuclear sites as a last resort.
"I have heard that there is currently a lot of clean-up going on at Parchin," one diplomat accredited to the IAEA said, who like others said such activity came to light only after the IAEA mentioned Parchin in a detailed report late last year.
Another diplomat told Reuters that he believed the Islamic Republic would not allow access to Parchin "unless they are extremely confident that there will be nothing found".
A U.S. think-tank, the Institute for Science and International Security (ISIS), said this month that satellite imagery of Parchin showed "what appears to be the final result of considerable sanitisation and earth-displacement activity".
Iran says Parchin, about 30 km (20 miles) southeast of the capital Tehran, is a conventional military site and has dismissed allegations aired about it as "ridiculous".
Friday's IAEA-Iran talks in Vienna could offer a last-minute chance for Tehran to influence the content of a pending agency report on Iran if Iranian authorities were to offer concessions regarding access to sites, documents and officials.
The latest quarterly report will be submitted to the agency's 35-nation governing board, which convenes on Sept. 10-14 with Iran likely to again dominate the agenda.
Amano said the IAEA had not drawn any conclusions yet.
"We are not saying Iran has nuclear weapons, we are not saying Iran has made a decision (to that end)," he said.
"(But) because pieces of information do indicate activities..., we would like Iran to engage with us to clarify these issues," he said, alluding to suspicions of possible military dimensions to Tehran's atomic programme.
The IAEA made clear in earlier rounds of talks this year that its overriding request is to go to Parchin.
Iran, for its part, says it must first agree a framework for the IAEA's inquiry before possibly allowing access - a stance dismissed by Western diplomats as delaying tactics.
Even if Iran did grant a visit to Parchin, U.N. inspectors would probably uncover no hard evidence of nuclear arms-related work, according to proliferation expert Mark Fitzpatrick at the International Institute for Strategic Studies think-tank.
"The clean-up probably could not totally remove uranium particles, but they wouldn't be enriched and Iran would be able to offer exculpatory explanations," Fitzpatrick told Reuters.