Vienna: Iran has not provided the International Atomic Energy Agency with sufficient information to substantiate its claim that its nuclear programme is peaceful, IAEA Director General Yukiya Amano said.
An Iranian official in Moscow later called the allegation "unfounded".
In his introductory speech to the IAEA Board of Governors, delivered in unusually strong language, Amano accused Iran of non-compliance with requests for information on its controversial nuclear programme and for inspections of its nuclear sites.
According to the latest IAEA report on Iran, published on the agency's website and dated Feb 21, 2013, the country has declared to the IAEA 16 nuclear facilities and nine locations outside facilities where nuclear material is customarily used.
"Iran is not providing the necessary cooperation to enable us to provide credible assurance about the absence of undeclared nuclear material and activities. The agency therefore cannot conclude that all nuclear material in Iran is used in peaceful activities," Amano said in his address to the 35-nation body.
He said talks with Iran "have been going around in circles".
A spokesman for the Iranian embassy in Moscow said the IAEA director's allegation was baseless.
"The production of nuclear material and the enrichment of uranium in Iran is monitored via IAEA video cameras, at all stages of production, 24 hours a day," said Moslem Chenari, head of the Iranian embassy's press service.
"Many of the claims (by the IAEA) lack clear evidence and are completely unfounded. Iran has openly cooperated with the IAEA," the diplomat added.
The IAEA director general, in his speech, also accused Iran of violating IAEA and UN Security Council resolutions by boosting its reserves of enriched uranium and the number of centrifuges.
According to the IAEA report, the country has installed 12,669 centrifuges to enrich uranium at its biggest nuclear enrichment plant in Natanz as of Feb 19, 2013, up 2,255 since the previous report.
In addition, 180 advanced centrifuges were also installed in the reported period, but are not yet operational. The number of centrifuges at another facility, Fordow, has remained constant since February last year, at 696.
The Institute for Science and International Security (ISIS), a non-profit organisation reporting on nuclear security issues, said 13,555 centrifuges were installed in Natanz as of May 15, up 886 since February.
"The number of centrifuges installed by Iran also continues to increase, as does the amount of enriched uranium it holds. These activities are in clear contravention of resolutions adopted by the (IAEA) Board of Governors and the United Nations Security Council," Amano said.
The UN Security Council adopted six resolutions as part of international efforts to address Iran's nuclear programme between 2006 and 2010.
Amano also reiterated his call to Iran "to provide access to the location at the Parchin site of interest to the agency", where the IAEA has tried in vain to get access in the past years.
The IAEA believes that a large explosives containment vessel was built at the Parchin site southeast of Tehran in 2000, and that it might be used to conduct experiments related to a nuclear weapons programme.
These activities are "strong indicators of possible nuclear weapon development", the agency said.
Iran maintains that the base in Parchin is a defence object and is therefore of no interest to the IAEA.
The IAEA chief told reporters later that because of suspected cleanup work at Parchin, it may no longer be possible for IAEA inspectors to obtain the required data even if they were granted access.
He said "massive removal of soil, asphalting and possible dismantling of infrastructure" were spotted at the site after the agency requested access there in January 2012.
"Because of the extensive activities by Iran it may no longer be possible to find anything (at Parchin)," he said.
"However, I still believe it is necessary for us to have access to the site because by visiting the site we can learn a lot of things."
Western powers say Iran is attempting to build an atomic weapon, but Tehran insists its programme has entirely peaceful purposes and has decried international pressure as unacceptable interference in its sovereign affairs.
So far, negotiations with Iran have focused largely on uranium enrichment issues.
However, analysts warn that another nuclear facility - the Arak Heavy Water Production Plant - may provide Iran with weapons-grade plutonium, an alternative ingredient for a nuclear bomb.
Tehran says Arak was designed to produce isotopes for medical and agricultural use, and that it has no intention to reprocess spent fuel from the reactor into plutonium for weapons.
Amano also mentioned Arak in his speech.
"The lack of up-to-date design information on the IR-40 Reactor at Arak is having an increasingly adverse impact on our ability to effectively verify the design of the facility and to implement an effective safeguards approach," he said.