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.

But Iran's critics were concerned that staunch opposition by nonaligned nations that traditionally back Iran in its nuclear dispute with the West would doom chances of consensus and lead to a vote, said diplomats. Because that could produce divisive debate and further polarization, it was unclear whether the resolution would be put forward.

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."

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.