Satellite images indicate buildings are being demolished and soil removed at Parchin, an Iranian military site the UN nuclear watchdog wants to visit, its chief said on Monday.
Yukiya Amano's comments will reinforce Western diplomats' suspicions that Iran is trying to remove any incriminating evidence from the Parchin facility before possibly granting the International Atomic Energy Agency (IAEA) access.
Amano, IAEA director general, said he hoped his agency and Iran would soon finalize an agreement enabling UN nuclear inspectors to resume a long-stalled investigation into suspected nuclear weapons research in the Islamic Republic.
The two sides will hold a new round of talks in Vienna on June 8, Amano said on the opening day of a week-long meeting of the IAEA's 35-nation governing board.
The agency's immediate priority in its investigation is to visit Parchin, where it believes Iran may have carried out high explosives tests that could be used in developing nuclear weapons.
Parchin, which Iran says is a conventional military complex, is at the center of Western allegations that Iran has conducted experiments - possibly a decade ago - that could help it develop nuclear bombs. Iran denies any such ambition.
Last week, a US think-tank published satellite images of Parchin which it said underscored concern that Iran is trying to destroy evidence of possible nuclear weapons-related research.
The Institute for Science and International Security (ISIS) posted the pictures on its website after the IAEA showed diplomats at a closed-door briefing similar images that Western envoys said suggested a clean-up at Parchin.
"The satellite imagery indicates that these activities include the use of water, demolishing of buildings, removing fences and moving soil," Amano told a news conference.
"These are some of the activities that we have observed through satellite imagery," he said, expressing concern that they could hamper the agency's efforts to find out what has been going on at the site, if and when it gains access.
Western diplomats say the buildings that appear to have been razed recently are small side buildings near the main structure that is of interest to the IAEA.
Ahmadinejad: Arrogant powers trying to thwart Iran's progress
Amano made a rare visit to Tehran two weeks ago and said when he returned to the Austrian capital that he expected a framework cooperation deal with Iran to be signed soon.
Iranian officials have made clear that only after reaching this kind of deal will they allow UN inspectors to visit Parchin, where the IAEA suspects Iran built a steel containment vessel in which to carry out the explosives tests.
Amano said both sides had shown flexibility in previous meetings and "we have narrowed down the differences ... it is close" to be agreed.
The IAEA says Tehran has stonewalled its investigation for almost four years, and Western diplomats have voiced doubt that Iran will implement any agreement that is reached.
They say Iran may be offering increased cooperation with the IAEA as a bargaining chip in its talks with world powers on ending the decade-old standoff over Tehran's nuclear program, an impasse that has led to the imposition of increasingly tough economic sanctions on Iran and fears of a new Middle East war.
"I think they are just stalling for time," one Western diplomat said about Iran.
Iran and the six powers - the United States, France, Russia, China, Germany and Britain - will meet for a third time this year in Moscow on June 18-19 after making scant progress on the dispute at their last meeting in Baghdad last month.
A new IAEA agreement with Iran could "greatly empower the diplomatic process," if the UN agency does not sign away its rights to rigorously investigate any leads that Iran is carrying out undeclared nuclear activities, a think-tank said.
For the UN Security Council to permit Iran to enrich uranium without facing continued sanctions, it must be confident that Iran is not working on the development of nuclear weapons, the Carnegie Endowment for International Peace added.
Amano told the board that Iran was not providing the cooperation needed to enable the IAEA to give "credible assurance about the absence of undeclared nuclear material and activities" in the country.
Expressing defiance, Iranian President Mahmoud Ahmadinejad said the "nuclear issue is part of the excuses and obstacles that arrogant powers impose on Iran," Iran's ISNA news agency reported.
"They try to prevent Iran from making progress in any possible way," he told a group of Egyptian journalists.