The UN nuclear watchdog has expressed concern about Iran's nuclear activities in a new report detailing what it calls "credible" information that Tehran may have worked on developing nuclear weapons.
In the report, published on Tuesday, the International Atomic Energy Agency (IAEA) said: "The agency has serious concerns regarding possible military dimensions to Iran's nuclear programme.
"After assessing carefully and critically the extensive information available to it, the agency finds the information to be, overall, credible. This information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."
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The agency also said that its information revealed that "prior to the end of 2003, these activities took place under a structured programme, and that some activities may still be ongoing".
The Vienna-based agency said it possessed information on Iran's work "on the development of an indigenous design of a nuclear weapon including the testing of components".
The IAEA, whose board could decide to report Tehran to the UN Security Council next week, called on Iran "to engage substantively with the agency without delay for the purpose of providing clarifications."
Iran, which says its nuclear programme is peaceful and which has been hit by four rounds of UN Security Council sanctions, dismissed the new IAEA report prior to its publication, saying it was based on false information.
The report comes amid rising speculation that Israel might launch a pre-emptive military strike in an attempt to knock out Iranian nuclear facilities.
Russian 'serious doubts'
Russia criticised the report, saying it would reduce hopes for dialogue with Tehran and suggested it was aimed at scuttling the chances for a diplomatic solution.
"We have serious doubts about the justification for steps to reveal contents of the report to a broad public, primarily because it is precisely now that certain chances for the renewal of dialogue between the 'sextet' of international mediators and Tehran have begun to appear," the Russian foreign ministry said in a statement.
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Russia and China had jointly pressured the IAEA not to even publish the report, diplomats in Vienna said.
Al Jazeera's Patty Culhane, reporting from Washington DC, said: "This is a report the US wanted the IAEA to come out with. We expect the Obama administration to use this report on the international stage to impose stricter sanctions... but to get that, they need China and Russia to get on board."
A senior US administration official told Al Jazeera: "The IAEA report does not assert that Iran has resumed a full-scale nuclear weapons programme, nor does it have a conclusion about how advanced those activities are, but clearly indicates there are activities of concern."
"I think going forward… this report will further underscore that Iran is the only [Non-Proliferation Treaty] signatory that is unable to convince the IAEA of the peaceful intent of its programme.
"That, in and of itself, further isolates Iran within the international community. I think it's incumbent on Iran to answer the very serious questions raised by this report."