The U.N. nuclear agency said Friday that it had failed to reach agreement with Iran on gaining access to suspected atomic research sites, dampening hopes for a breakthrough during high-level nuclear talks scheduled this month.

In an unusually blunt statement, officials of the International Atomic Energy Agency called the outcome of Friday’s meeting “disappointing” and said Iran appeared to retreat from commitments it had made during earlier meetings in the Iranian capital.

“There has been no progress, and indeed Iran raised issues that we have already discussed and added new ones,” Herman Nackaerts, the IAEA deputy director general, said after the talks concluded in Vienna.

No date was set for future negotiations, which were aimed at clearing up a years-long dispute over allegations of secret nuclear weapons research conducted by Iranian scientists nearly a decade ago.

The IAEA has been particularly eager to visit Iran’s Parchin military base, where Iranian scientists are alleged to have tested explosive triggers for nuclear weapons inside a large, tanker-shaped test chamber.

Iran says such experiments--detailed in documents obtained by Western spy agencies--never occurred, but it has refused to allow IAEA officials near the site since 2005. Satellite photos in recent weeks have shown major renovation underway at the site, with several buildings razed and soil removed near the place where the alleged test chamber once stood. Iran has consistently said that its nuclear program is for peaceful purposes, not making weapons.

The setback in negotiations occurred less than three weeks after IAEA officials claimed success in persuading Iran to grant its inspectors access to key facilities and scientists said to have been involved in the weapons research. IAEA chief Yukiya Amano, after traveling to Iran on May 20, said the agency expected to sign a formal agreement within days spelling out the terms of the deal.

Since then, nuclear diplomacy with Iran has taken a negative turn. On May 24, Iran balked at a plan offered by six world powers that called for strict limits on Iran’s nuclear activities, in part to assuage concerns that Tehran is seeking a nuclear weapons capability. And Iranian leaders have struck a defiant tone this week in public statements about an upcoming round of negotiations set to begin June 18 in Moscow. The talks will be the third this year between Iran and the P5-plus-1 block, which consists of the United States, Britain, China, France, Germany and Russia.

“Their policies of pressure and intimidation are futile,” Iranian Foreign Minister Ali Akbar Salehi said Wednesday at a news conference in Tehran. “They have to adopt policies to show goodwill to solve this issue.”

The six powers are pressing Iran to halt production of a more purified type of enriched uranium that can be easily converted into weapons-grade uranium for bombs. But Iran has insisted that it must be granted immediate relief from economic sanctions as part of any agreement.

On Friday, China unexpectedly added to the pressure on Iran by publicly calling on its leaders to compromise. While hosting Iranian President Mahmoud Ahmadinejad in Beijing, Chinese leader Hu Jintao urged Tehran to take practical steps to ease international tensions over its nuclear program.

“China hopes the Iranian side can weigh up the situation, take a flexible and pragmatic approach, have serious talks with all six related nations, and enhance dialogues and cooperation with the International Atomic Energy Agency so as to ensure the tensions can be eased through negotiations,” Hu said, according to a statement released by China’s Foreign Ministry.