The head of the United Nations nuclear watchdog, Yukiya Amano, said Monday that he was "unable to report any progress" in the organization's talks with Iran over its atomic program.

The Vienna-based International Atomic Energy Agency has been trying for more than a year to persuade Iran to cooperate with a long-stalled agency investigation into suspected atom bomb research by the Islamic state, which denies any such activity.

Amano said the agency remained committed to engaging in constructive dialogue with Iran, but that negotiations must proceed with "a sense of urgency and a focus on achieving concrete results" soon.

The IAEA's priority is to be able to inspect Parchin, a sprawling site southeast of the capital Tehran, where it believes Iran built an explosives chamber to carry out tests, possibly a decade ago. Iran denies this.

"Providing access to the Parchin site would be a positive step which would help to demonstrate Iran's willingness to engage with the agency on the substance of our concerns," Amano said, according to a copy of his speech.

Iran was upbeat last week after talks with world powers about its nuclear work ended with an agreement to meet again, but Western officials said it had yet to take concrete steps to ease their fears about its atomic ambitions.

During the talks in the Kazakh city of Almaty, the United States, China, France, Russia, Britain and Germany offered modest sanctions relief in return for Iran curbing its most sensitive nuclear work, but made clear that they expected no immediate breakthrough.

Meanwhile, US Secretary of State John Kerry warned Monday there was a limited period of time available for talks between Iran and major powers about its nuclear ambitions.

"There is a finite amount of time," Kerry, on a visit to Saudi Arabia, said of the talks between Tehran and a group of world powers. He was speaking at a news conference held jointly with his Saudi counterpart Prince Saud al-Faisal.

Prince Saud suggested that the Iranians did not show enough seriousness in the Almaty talks.

"We can't be like philosophers who keep talking - We have to talk seriously and honestly and we have to put in our commitment clearly on the table," he said.

"Negotiations cannot go on forever," he added.

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