The US has accused Iran of stonewalling calls for scrutiny of its nuclear programmes and says it is "deeply troubled" by Tehran's plans to start a reactor in 2014 that could yield bomb material.
A European Union statement described Iran's "procrastination" as unacceptable and US envoy Joseph Macmanus said the lack of progress documented Iran's longstanding practice of deception and non-compliance.
"We are deeply troubled that Iran claims that the IR-40 heavy water reactor at Arak could be commissioned as soon as early 2014, but still refuses to provide the requisite design information for the reactor," the US ambassador to the International Atomic Energy Agency (IAEA) told the 35-nation Board of Governors on Wednesday, according to a copy of his speech to the closed-door gathering.
Macmanus cited IAEA rules that a member state must inform the Vienna-based UN agency about a nuclear plant, and give design details, as soon as it has decided to build one. Iran has said it must only do so before loading nuclear fuel into the reactor.
"Iran's refusal to fulfil this basic obligation must necessarily cause one to ask whether Iran is again pursuing covert nuclear activities," Macmanus said.
The UN probe has been stalled for more than six years and 10 rounds of Iran-IAEA negotiations over the past 18 months have failed to revive it.
EU 'deeply concerned'
Iran has said all of its nuclear programmes are peaceful and that the allegations of weapons work are based on forged intelligence from its enemies.
The EU said it was deeply concerned about Iran's enriched uranium programme and the reactor in at Arak, central Iran.
Iran had yet to respond to the latest accusations.
An earlier statement by the US and five other world powers was less forceful, reflecting the sensibilities of China and Russia.
While joined with the United States, Britain, France and Germany in trying to negotiate with Iran on its nuclear programme, those two nations are traditionally less critical of Tehran for strategic and economic reasons.
The six said it was essential and urgent that Iran reached an agreement with the IAEA that would allow the agency to investigate suspicions Iran worked on nuclear weapons.
They said IAEA access to Parchin was particularly important.
Reactor delivery report
Beyond fears that Tehran might have worked on atomic arms, most international concern has focused on its uranium enrichment, because this was further advanced than the reactor and already had the capacity to enrich uranium to weapons-grade.
The IAEA said in a May report that Iranian technicians at Arak, in central Iran, had already taken delivery of a huge reactor vessel to contain the facility's fuel.
It detailed progress in Tehran's plans to test the fuel.
Olli Heinonen, the former IAEA Deputy Director General, said that with such progress, there was a good possibility that the reactor would have started to operate by the end of 2014.
He said Iran had been successful in evading sanctions in importing some components for Arak, while apparently manufacturing others domestically.
The IAEA's ability to monitor the reactor to make sure none of its plutonium is being diverted for possible weapons use was another worry.
The agency said it had all the means it needed to make sure that did not happen with Iran's enriched uranium, including cameras, physical inspections and seals on certain materials and components.
But it complained that Tehran's refusal to provide construction details of the reactor could hamper it in looking for possible proliferation dangers there.
Heinonen, the former IAEA official, said the agency could devise proper monitoring methods and equipment at any facility it was supervising only if it saw the plans early.