(Reuters) -

U.N. nuclear investigators may no longer find anything if granted access to Iran's Parchin military site, their chief said on Monday, in view of suspected Iranian efforts to remove any traces of illicit atomic activity there.

Yukiya Amano also said his agency's talks with Iran on unblocking an IAEA inquiry into possible nuclear arms research by Tehran had been "going around in circles" for some time.


Amano was airing unusually blunt criticism that reflected the mounting tension over Iran's disputed nuclear energy program that has increased fears of a new Middle East war.

Israel, widely assumed to be the Middle East's only nuclear-armed state, sees Iran's declared civil nuclear program as the most serious risk to its security and has threatened air strikes if diplomacy and sanctions fail to rein in Tehran.

Amano, director general of the International Atomic Energy Agency, vented growing frustration at the lack of results in getting Iran to address international concerns. Tehran denies its nuclear energy quest is a disguised bid for atomic bombs.

In hard-hitting comments to the IAEA's 35-nation Board of Governors and later at a news conference, he also said Iranian advances in building a research reactor and in its uranium enrichment work were in "clear contravention" of U.N. Security Council resolutions calling for a suspension in such activities.

The IAEA has been trying since early 2012 to engage with Iran over what the Vienna-based U.N. agency calls the "possible military dimensions" to the country's nuclear program.

But 10 rounds of negotiations in the last 17 months have failed to achieve any breakthrough. Western diplomats accuse Iran of stonewalling the IAEA, an allegation Tehran rejects.

"To be frank, for some time now we have been going around in circles," Amano, a veteran Japanese diplomat, said.

The IAEA's priority has been to gain access to Parchin, a sprawling military compound where it believes Iran may have carried out explosives tests applicable to developing a nuclear weapon, possibly a decade ago. Iran denies this.

But Amano acknowledged for the first time that "extensive activities" by Iran - including removal of soil and asphalting - now meant inspectors may return empty-handed even if Iran were to allow them to visit. Iran says Parchin is a conventional military site and has dismissed the cleansing allegations.

"It may no longer be possible to find anything," he said, adding, however, that the IAEA still wanted to go to Parchin.

IRAN REACTOR WORRIES WEST

Amano spoke at a time of apparent deadlock in a broader diplomatic initiative by six world powers to find a peaceful solution to the decade-old dispute over Iran's nuclear ambitions. Western diplomats say they are awaiting the outcome of Iran's June 14 presidential election but still do not anticipate any notable rollback from its nuclear defiance.

Iran, a big oil producer now under harsh Western sanctions against its lifeblood export sector, says its nuclear program aims to meet the electricity needs of a rapidly growing population and advance some areas of scientific research.

But its refusal to suspend nuclear activity with both civilian and potential military applications in defiance of U.N. Security Council demands, and its lack of full openness with the IAEA, have fuelled suspicions abroad about its ultimate goals.

"Iran is not providing the necessary cooperation to enable us to provide credible assurance about the absence of undeclared nuclear material and activities," Amano said.

Western and Israeli worries about Iran are focused largely on its uranium enrichment work, as such material refined to a high level can provide the fissile core of an atomic bomb.

But diplomats and experts say a heavy water research reactor being built near the town of Arak could give Iran an alternative ingredient - plutonium - for nuclear bombs, if it were to decide to build such weapons of mass destruction.

An IAEA report issued to member states last month showed the Islamic Republic pressing ahead with the construction of Arak, including the delivery to the site of the reactor vessel.

"Iran continues to advance its heavy water-related projects," Amano said. The lack of updated design information about the plant "is having an increasingly adverse impact on our ability to ... implement an effective safeguards approach."

(Editing by Mark Heinrich)

By Fredrik Dahl