The International Atomic Energy Agency (IAEA) said in a report released late Monday that Iran must provide "substantive" information if it is to convince the international community that its nuclear drive is peaceful.
The alleged studies suggest Iran may have been trying to develop a nuclear warhead, convert uranium and test high explosives and a missile re-entry vehicle, said the report.
The report, to be discussed by the IAEA's board of governors at a June 2-6 meeting, said intelligence from a number of sources suggest Iran has conducted the studies. Iran has repeatedly dismissed the allegations as "baseless" and the intelligence as "forged".
The IAEA demanded that Iran, which already faces UN Security Council sanctions over its refusal to halt uranium enrichment, disprove the allegations.
"Substantive explanations are required from Iran to support its statements on the alleged studies and on other information with a possible military dimension," the report insisted.
"The alleged studies … remain a matter of serious concern. Clarification of these is critical to an assessment of the nature of Iran's past and present nuclear programme."
The IAEA "is continuing to assess the information and explanations provided by Iran. However, at this stage, Iran has not provided the agency with all the information, access to documents and access to individuals necessary to support Iran's statements."
The IAEA "is of the view that Iran may have additional information, in particular on high explosives testing and missile-related activities, which could shed more light on the nature of these alleged studies and which Iran should share with agency."
The watchdog said Iran is still refusing to suspend uranium enrichment, the process that can be used to make fissile material for an atomic bomb, despite three rounds of UN sanctions.
In all, Iran was operating about 3,500 uranium-enriching centrifuges at its main nuclear site in Natanz, the report said.
Tehran has told the IAEA it hopes to have some 6,000 centrifuges running within months, a target that agency experts say is achievable, a senior official close to the Vienna-based watchdog said.
US and European officials said the report showed that Iran was continuing to stonewall the agency's long-running investigation into its atomic drive.
"The report shows in great detail how much Iran needs to explain, and how little it has," said the US ambassador to the IAEA, Gregory Schulte.
The IAEA's director general Mohamed ElBaradei "does not report progress, he does not report cooperation," Schulte added.
"It's disappointing, although perhaps not surprising, given Iran's long history of evasiveness over the real purpose of its nuclear programme," said an EU diplomat.
Nevertheless, Iran's ambassador Ali Asghar Soltanieh insisted his country had answered all outstanding questions.
"We have left no question unanswered. We have given all the explanations needed as far as we are concerned," Soltanieh told AFP in a telephone interview.
"We have given 200 pages of explanations" with regard to the so-called weaponisation allegations, Soltanieh continued. "We have dealt with this in depth."
The Iranian envoy insisted the new report was "a vindication and reiteration of the peaceful nature of Iran's nuclear activities."
Iran would not suspend enrichment, Soltanieh said. "We will continue enrichment, while not suspending our cooperation with the IAEA."
EU foreign policy chief Javier Solana said Monday he hoped to travel to Iran soon to present a new offer from major world powers on suspending enrichment.
Solana said in Brussels that he met Iran's Foreign Minister Manouchehr Mottaki in Lebanon on Sunday but had no date for his proposed trip.
"We have a new proposal. I would like very much to present it to them," Solana said. Permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany have re-worked an offer of incentives originally made in 2006.
