VIENNA (AFP) - The UN atomic watchdog seems to be taking a tougher line with Iran on its nuclear programme, accusing it of withholding key information on alleged attempts to make nuclear arms, experts said Tuesday. The International Atomic Energy Agency is "clearly frustrated by Iran's stonewalling. They want answers," said Joe Cirincione, non-proliferation expert and head of the Washington-based Ploughshares Fund, following the release of the IAEA's latest Iran report late Monday. The restricted report, a copy of which was obtained by AFP, has been forwarded by the IAEA to the UN Security Council and is due to be discussed by the watchdog's own 35-member board of governors next week. Using far stronger language than the past, the IAEA expressed "serious concern" that Iran is still hiding information about alleged studies into making nuclear warheads and defying UN demands to suspend uranium enrichment. The agency said it was "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 alleged studies comprise a uranium conversion project, high explosives testing and designs for a missile re-entry vehicle and suggest there may have been a possible military angle to past nuclear work. Mark Fitzpatrick, non-proliferation expert at the International Institute for Strategic Studies in London, noted the unusually strong tenor of the report. It was "harder on Iran, both in the detail and in the tone," Fitzpatrick said. There was no "sugar-coated suggestion that Iran has been forthcoming" in addressing the outstanding issues. Instead, the report "clearly refutes Iran's claims that it has done everything it needs to do with regard to the action plan." A senior official close to the Vienna-based watchdog confirmed there was growing frustration within the agency over Iran's attitude. "We have not got substantive answers and we could have got those earlier," the official said, speaking on condition of anonymity. "I don't see any reason why they cannot provide those" answers, he said. "They know what we need. The questions are clear. We need the answers." The US ambassador to the IAEA, Gregory Schulte, said Iran had made scant progress in clearing up the outstanding issues, while the French government said it believed the report "confirms the international community's concerns." German Foreign Minister Frank-Walter Steinmeier said the "leaves open a number of questions that we will have to examine very quickly." Iran "is still not demonstrating the spirit of cooperation," Steinmeier complained. "We have left no question unanswered. We have given all the explanations needed as far as we are concerned," Iran's ambassador to the IAEA, Ali Asghar Soltanieh insisted, told AFP. Nevertheless, for Daryl Kimball, executive director of the Arms Control Association in Washington, the IAEA's report was "just another in a series of bad signs." Meanwhile, Iran's refusal to respond to questions from the UN nuclear watchdog is "very troubling," a senior US official said Tuesday. "This is a case when the report really speaks for itself," State Department spokesman Sean McCormack said. "It is very troubling that the Iranian government refuses to answer questions about the military involvement in this program, which can raise a whole other set of issues about what were the activities vis-a-vis weaponising or trying to weaponise a nuclear program," he said. "So these questions remain unanswered. I think the report speaks for itself in that regard," he said.