Updated

The chief U.N. nuclear inspector urged Iran on Monday to stop stonewalling his experts and cooperate over indications it may have tried to make atomic arms under the cover of a civilian nuclear program.

A senior Iranian official said such allegations were "fabricated." But a U.S. envoy repeated Washington's assertions that Tehran tried to make such weapons in the past.

The exchange came as the 35-nation board of the International Atomic Energy Agency convened to focus on Iran's defiance of U.N. Security Council demands to freeze uranium enrichment. It is also looking at an IAEA report that says Iran has cleared up some questions about its nuclear past but that it refuses to discuss the topic of its alleged weapons development activities beyond dismissing them as groundless accusations.

Calling the issue of alleged weapons development, "a matter of serious concern," IAEA chief Mohamed ElBaradei, in comments to the board, urged Iran "to be as active and cooperative as possible in working with the agency" to put the issue to rest.

"A full-fledged examination of this issue has yet to take place," he said, indirectly denying Iranian suggestions that an IAEA probe of Tehran's past activities has ended positively.

As the board met in Vienna, the U.N. Security Council scheduled a session expected to agree on a third set of sanctions to punish Iran for ignoring its demand to freeze enrichment, a potential component of a nuclear weapons program.

A diplomat accredited to the IAEA said all five permanent council members appeared ready to back new sanctions as well as the majority of temporary members, with only Libya, South Africa and Indonesia opposed. He demanded anonymity because his information was confidential.

A separate draft resolution was in the works in Vienna. Britain, France and Germany prepared a document focused on Tehran's nuclear failings that would be supported by the majority of board members — including the United States, Canada, Australia, Japan and the EU.

A senior diplomat, also speaking on condition of anonymity because of confidentiality, said the resolution would be introduced by Wednesday. He said it would praise ElBaradei for his efforts, but note the investigation's failings on the issue of nuclear weapons and emphasize the board's authority on the issue of Iran's nuclear program.

Ali Ashgar Soltanieh, Iran's chief IAEA representative, dismissed any push for an IAEA resolution critical of his country as "ridiculous."

In comments later Monday, he dismissed intelligence forwarded to the IAEA by the U.S. and its allies on purported weapons programs by Tehran as "forged and fabricated," and driven by "political motivations." And he suggested that for Iran the issue was closed, saying the IAEA probe "is concluded."

But chief U.S. IAEA delegate Gregory L. Schulte said the report shows that "troubling questions remain about Iranian activities that strongly suggest a clandestine weapons related program." Both he and Soltanieh spoke to reporters.

Iran's nuclear past has been a key topic at IAEA board meetings for the past five years, after revelations that the Islamic Republic had carried out nearly two decades of secret atomic work, some of it potentially linked to weapons research.

The last IAEA resolution, more than two years ago, referred Tehran's nuclear file to the Security Council. The council imposed two sets of sanctions to punish Iran for its refusal to suspend uranium enrichment and fully open its nuclear activities to agency inspectors.

Iran not only continues enrichment — which can produce both nuclear fuel and the fissile core of nuclear warheads — but has expanded its activities, saying it has a right to the technology for a civilian nuclear program. That has led to Monday's planned council vote.

Additionally fueling Western frustrations is Tehran's refusal even to discuss allegations that it tried to make nuclear weapons through interlinked projects enriching uranium, testing explosives and designing missiles and warheads suitable for a nuclear payload.

While a Feb. 22 IAEA report said a probe had made progress on other past nuclear activities, it said that Tehran had not responded properly to intelligence forwarded by the U.S. and its allies purportedly showing nuclear weapons technology being developed by Iran.

Iran has focused on the agency's assessment of progress on some topics to assert that the IAEA report vindicates its claims that its nuclear project is strictly civilian. Soltanieh on Monday said the report "proves Iran's activities are peaceful."