VIENNA, Austria — The U.N. nuclear monitoring agency shared new photos and documents purporting to show that Iran tried to refit its main long-distance missile to carry a nuclear payload, said diplomats who attended the meeting today.

Responding to the presentation to the 35-nation board of the International Atomic Energy Agency, a senior American envoy said the information was compelling evidence against the Islamic Republic. His Iranian counterpart said the material shown was fabricated.

Other diplomats, who demanded anonymity because they were not authorized to comment on the closed meeting’s details, de described the information as credible but unverified.

The briefing focused on an IAEA report circulated to the board members yesterday that faulted Iran for blocking efforts to further investigate the alleged weapons program. The report also confirmed that Iran was expanding uranium enrichment activities — which can make both nuclear fuel and warhead payloads — despite three sets of U.N. Security Council sanctions.

Part of the report spoke of what appeared to be drawings and calculations by Iranian engineers on reconfiguring its Shahab-3 missile to be able to carry a nuclear payload, and the presentation today went into greater detail, the diplomats said.

Iranian officials say the new missile has a range of 1,250 miles, enabling a strike on Israel and most of the Middle East.

The presentation “showed board members for the first time photographs and documents of work undertaken in Iran on the redesigning of the Shahab-3 missile to carry what would appear to be a nuclear weapon,” the chief American representative to the IAEA, Gregory L. Schulte, said. He said the senior IAEA official doing the briefing “told us that information they have is very credible.”

The Iranian envoy said the IAEA determined that the material shown could not verified.

“We have given clear information … (on) why this material is fabricated,” Ali Ashgar Soltanieh told reporters in separate comments. He called for “an end to this endless process” of probing Iran for evidence of an arms program he said never existed.

A diplomat inside the meeting said the truth lay somewhere between American and Iranian standpoints, telling The Associated Press board members were told “the information is credible but cannot be verified.”

Another said that while the information was compelling, most of it was known and what was new in the presentation “appeared to be only a few new photos and diagrams.”

With the IAEA’s deputy director, General Olli Heinonen, the briefing was conducted by his aide, Hermann Naeckerts, who was “more cautious” than General Heinonen had been at an earlier briefing, he said.

At Washington, the State Department said it would host a meeting of top negotiators from the five U.N. Security Council countries and Germany on Friday to discuss how to proceed in wake of Monday’s IAEA report, which was also send to the council. Officials said the meeting would be held to prepare for a gathering of the six foreign ministers expected next week on the sidelines of the annual U.N. General Assembly session at New York.

America and its Western allies would like to see new U.N sanctions against Iran, but Russia and China traditionally oppose harsh Security Council action. China said today that further penalties will not resolve the nuclear impasse.

Still, a White House spokesman, Gordon Johdroe, spoke yesterday of “the possibility of new sanctions.” And today a French Foreign Ministry spokesman, Eric Chevallier, told reporters that Paris had “no other choice than to work in the days and weeks to come toward a new Security Council sanctions resolution.”

The IAEA’s latest report suggested Iran has now amassed a third of the amount of enriched uranium it could reprocess into the material for the fissile core of a nuclear weapon should it choose to do so. But U.N officials familiar with the report emphasized that Iran — whose known nuclear programs are under IAEA supervision — has shown no indication it wanted to go that route.

America and its allies allege that Iran wants to develop its uranium enrichment program to make nuclear weapons. But oil-rich Iran insists it only wants to make nuclear energy, and IAEA oversight and inspections of its known enrichment program has not found any evidence that contradicts that.

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Associated Press writers Matthew Lee in Washington and Jamey Keaten in Paris contributed to this report.