(Reuters) - The United Nations nuclear watchdog signaled on Monday it would press Iran for access to a military installation where it suspects Iran has built a chamber for high-explosive tests that could serve to develop atomic bombs.

The Vienna talks will test Iran´s readiness to address U.N. inspectors´ suspicions of military dimensions to its nuclear program, ahead of broader-ranging talks on the program´s future in Baghdad next week between Tehran and six world powers.

Iran, which rejects Western accusations it seeks nuclear arms, has so far resisted requests by the International Atomic Energy Agency to visit the extensive Parchin complex southeast of Tehran. The issue was expected to be raised during a high-level May 14-15 meeting in Vienna between Iran and the IAEA.

"It is important now ... that Iran let us have access to people, documents, information and sites," IAEA Deputy Director General Herman Nackaerts told reporters as he arrived for the talks at an Iranian diplomatic mission in Austria´s capital.

His team of senior IAEA officials and experts left the building after about five hours, declining any comment to media waiting outside. The meeting will resume on Tuesday.

Iranian state television said: "The first round of talks has been evaluated as positive." It did not elaborate.

An IAEA report last November found that Iran had built a large containment vessel in 2000 at the Parchin site in which to conduct tests that the U.N. agency said were "strong indicators of possible (nuclear) weapon development".

It said a building was constructed "around a large cylindrical object". An earth berm between the building containing the cylinder and a neighboring building indicated the probable use of high explosives in the chamber.

The IAEA said it had obtained satellite images that were consistent with this information. The vessel was designed to contain the detonation of up to 70 kg of high explosives.

Israel - widely believed to hold the Middle East´s only nuclear arsenal - and the United States have not ruled out military action to prevent Iran from obtaining atomic bombs if negotiations fail to achieve this goal peacefully.

Western diplomats say they suspect Iran is now cleaning the Parchin site to remove incriminating evidence. A U.S. security institute said last week that satellite imagery showed activity there which it said raised concern that Iran may be "washing" the building the IAEA wants to see.

A Western diplomat told Reuters he had seen other images also suggesting a clean-up operation at Parchin, including a stream of water apparently coming from the building.

Iran´s Foreign Ministry spokesman has dismissed the allegations, saying nuclear activities cannot be washed away.

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But the Institute for Science and International Security (ISIS), the Washington-based think-tank which published the satellite image last week, said this was incorrect.

"The concern is that washing could be incorporated into an effort to cleanse the building. The process could involve grinding down the surfaces inside the building, collecting the dust and then washing the area thoroughly. This could be followed with new building materials and paint," it said.

A senior Iranian lawmaker, Alaeddin Boroujerdi, did not rule out a possible IAEA visit to Parchin. "Naturally, agreements are always reached behind the negotiating table and parliament will respect any agreement reached by Iranian representatives," he told ISNA news agency when asked about the U.N. body´s request.

Nackaerts, head of the IAEA´s nuclear inspections worldwide, said Tehran must now engage on substance with the agency in its nuclear investigation, after years of stonewalling.

Two previous rounds of talks in Tehran this year with U.N. inspectors failed to make any notable progress, especially on their request to go to Parchin.

"The aim ... is to reach agreement on an approach to resolve all outstanding issues with Iran," Nackaerts said. "In particular, clarification of the possible military dimensions remains our priority."

Nackaerts did not name any sites, but IAEA Director General Yukiya Amano said earlier this month that gaining access to Parchin would be the priority for the IAEA in the talks.

"Some IAEA officials see Tehran´s refusal of access as a challenge to the IAEA´s primacy in setting the agenda for inspections, and for that reason the IAEA will continue to request access to that site as a matter of principle," said Mark Hibbs of the Carnegie Endowment for International Peace.

Western diplomats will be watching the discussions for any sign that Iran is now ready to make concrete concessions, saying this would send a positive message ahead of the Baghdad talks.

Iran and the powers involved in nuclear diplomacy - the United States, Russia, China, Britain, France and Germany - revived negotiations in Istanbul last month after a 15-month hiatus and both sides say they hope for progress in Baghdad.

The resumption of diplomacy offers a chance to defuse tension that has led the United States and the European Union to try to block Iran´s oil exports through sanctions, and increased worries about a new Middle East war.

UK WARNS ON SANCTIONS

The West suspects Iran is seeking to develop the capability to make nuclear bombs, although intelligence officials believe Tehran has not made a decision whether to actually build them.

The Islamic Republic, one of the world´s largest oil producers, says its atomic program is a peaceful quest to generate more electricity for a rapidly growing population.

Iran "will not retreat even one iota from its fundamental rights," Iranian media quoted President Mahmoud Ahmadinejad as saying, showing traditional defiance in the face of Western demands on Iran to curb the nuclear program.

British Foreign Secretary William Hague said the European Union wanted to see "concrete steps and proposals" from Iran.

"Without that, of course we have sanctions we have imposed. They will not only be enforced but, over time, intensified," he told reporters before a meeting of EU foreign ministers.

Iran has suggested that a broader agreement with the IAEA - which regularly monitors Iran´s declared nuclear sites - on how to address outstanding questions should be reached before it would consider letting inspectors into Parchin.

Western diplomats see this as a stalling tactic and do not expect Iran suddenly to allow access to Parchin.

A Western priority is for Iran to halt the higher-grade uranium enrichment work it started two years ago and has since expanded, potentially shortening the time needed to build a bomb. Iran wants the Baghdad meeting to yield a deal on an easing of sanctions, something the West will be reluctant to consider before seeing substantive concessions.

Refined uranium can be used to fuel nuclear power plants, which are Iran´s stated goal, or provide material for bombs if processed further, which the West suspects is the country´s ultimate intention.

(Additional reporting by Zahra Hosseinian; Editing by Mark Heinrich) (© Thomson Reuters 2012. 05/14/12)