The UN nuclear watchdog and Iran failed Friday to agree a deal allowing greater access to Tehran's contested nuclear programme, a setback as world powers prepare for crucial talks in Moscow.

"There has been no progress," IAEA chief inspector Herman Nackaerts announced late Friday after all-day talks with Iran's envoy to the agency Ali Asghar Soltanieh and IAEA deputy director general Rafael Grossi.

"This is disappointing," he said, reading out a prepared statement at a joint briefing with the Iranian ambassador.

The International Atomic Energy Agency had come to the meeting "in a constructive spirit and with the desire and intention of finalising the paper" but Iran had imposed new conditions on a deal, Nackaerts said.

A date for a new meeting between the two sides had yet to be set, he added.

The agency has been seeking a deal with Iran that would allow greater access to sites, people and documents tied to Tehran's nuclear programme.

After a visit to Iran on May 21, IAEA chief Yukiya Amano had said the two sides were close to a deal.

However, Soltanieh appealed Friday for "time and patience" and vowed that Iran was "ready to remove all ambiguities and prove to the world that our activities are exclusively for peaceful purposes."

"Let Iran and the IAEA do their work," he said, adding that "there is no obstacle" to an accord being struck at a later date.

The planned accord would include agency access to the Parchin military base near Tehran, where the IAEA believes suspicious explosives testing was carried out before 2003 and possibly after that.

On two visits to Iran, in January and February, the UN nuclear watchdog said it was denied access to the site.

Meanwhile, new satellite imagery indicated "extensive activities" at the base, which "could hamper the agency's ability to undertake effective verification" of the site, the IAEA said in a report last month.

In other words, experts spoke of a clean-up, pointing to the razing of two small buildings and what looked like a water run-off.

But Soltanieh dismissed this as politicisation by Western countries.

"Whoever raises the issue of Parchin or other sites which is going to be dealt with in this framework... is just creating a negative environment, and this is not advisable and this is not conducive," he told journalists.

Western powers and Israel suspect Iran is trying to produce a nuclear bomb, but Tehran insists it is merely developing atomic power for civilian purposes.

Earlier this week, IAEA chief Yukiya Amano again warned Iran that it needed to do more to alleviate international concerns over its nuclear drive.

"Iran is not providing the necessary cooperation to enable the agency to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities," he said.

The negotiations with the IAEA come ahead of a crucial new round of talks between Iran and the so-called P5+1 world powers -- the United States, Russia, China, Britain, France and Germany -- in Moscow on June 18-19.

Barring progress there, an EU oil embargo against Iran will come into force on July 1, adding to a range of sanctions imposed under UN resolutions.

During a meeting in Beijing Friday, China's President Hu Jintao urged his Iranian counterpart Mahmoud Ahmadinejad to be "flexible and pragmatic" ahead of Moscow and to cooperate with the IAEA, Xinhua state news agency reported.

Talks with the P5+1 were revived in Istanbul in April after being stalled for a year, and were held again in Baghdad in May, although little was achieved.

A key source of dispute has been Iran's enrichment of uranium to 20-percent purity, bringing Tehran consistently closer to producing the 90-percent enriched uranium required to make a bomb, according to Western powers.

On Thursday, Tehran insisted that Western powers must recognise its "right" to uranium enrichment -- which it says it needs to produce medical isotopes -- if talks in Moscow are to advance.