WASHINGTON — International nuclear inspectors reported on Friday that Iran was moving more rapidly to produce nuclear fuel than many outsiders expected, at a deep underground site that Israel and the United States have said is better protected from attack than Iran’s older facilities.

The report by the International Atomic Energy Agency indicated that for the first time, Iran had begun producing fuel inside the new facility, in a mountain near the holy city of Qum. The agency’s inspectors found in their most recent visits that over the past three months, Iran had tripled its production capacity for a more purified type of fuel that is far closer to what is needed to make the core of a nuclear weapon.

The report is likely to inflame the debate over whether Iran is nearing what Israel’s defense minister, Ehud Barak, calls entering a “zone of immunity.” The phrase refers to a vaguely defined point beyond which Iran could potentially produce weapon fuel without fear of an air attack that could wipe out its facilities.

The Iranians showed the inspectors the progress they had made at the underground facility, also known as Fordo, as part of the regular inspection of declared nuclear sites. They seemed eager to demonstrate that despite sanctions, sabotage and several United Nations Security Council resolutions, they were forging ahead in building a facility with a capability they insist is purely for energy production and medical research. But the Iranians know that this facility, under 250 feet of granite, is the one that worries Israel and the West the most, and the resources that Iran is putting into equipping it leaves considerable ambiguity about their intent.

For years, the Iranians have refused to answer questions raised by the inspectors about what the I.A.E.A. delicately calls “possible military dimensions” of the Iranian program — evidence that some work has been conducted on warhead designs, trigger devices and similar technologies that strongly suggest that the country is contemplating using its fuel for weapons.

The White House, which has been trying to increase the economic pressure on Iran while trying to dissuade Israel from attacking Iran’s nuclear facilities, characterized the newest report as more evidence of Iranian defiance. “Iran has continued to pursue its uranium enrichment program in violation of multiple United Nations Security Council resolutions without demonstrating any credible or legitimate purpose for doing so,” the National Security Council spokesman, Tommy Vietor, said on Friday. “When combined with its continued stonewalling of international inspectors, Iran’s actions demonstrate why Iran has failed to convince the international community that is nuclear program is peaceful.”

Despite the I.A.E.A.’s findings at Fordo, American officials insist that Iran’s overall progress has been halting at best. The report also shows that despite Iran’s repeated boasts, it is still having trouble using a significant amount of next-generation equipment to make fuel. The United States also argues, in anonymous interviews and in conversations with Israeli officials, that Iran’s program has a number of vulnerabilities that could be exploited should it decide to try to develop a bomb. American intelligence officials say they do not believe that Iranian leaders have made that decision, though Israeli and British intelligence disagree.

When President Obama and other Western leaders first made public the discovery of the new facility in 2009, American officials said they believed that its exposure meant it would never be used. However, the report on Friday indicated that 696 centrifuges — the tall, silvery machines that enrich uranium by spinning it at supersonic speeds — have been installed. An additional 2,088 have been partially installed, meaning the facility is approaching its design capacity.

The 11-page report also described how Iran has refused, in two separate meetings with inspectors, to answer questions raised in the I.A.E.A.’s last report, issued in November, about experiments that could be linked to work on nuclear weapons. Inspectors were told they could not visit a military site called Parchin, where the inspectors suspect that work was done on conventional explosives that can be used to trigger a warhead. “Iran stated that it was still not able to grant access to that site,” the report said.

Iran has said that it produces fuel enriched to 20 percent purity, the highest level the I.A.E.A. reported being produced at Fordo, to replenish a small nuclear reactor in Tehran that is used to make medical isotopes. That claim appears to be true, at least in part: the inspectors say a fraction of the fuel was used to manufacture a single fuel assembly that was inserted in that reactor in recent days as President Mahmoud Ahmadinejad watched. The event was televised in Iran, underscoring the country’s intent to continue its nuclear program despite international sanctions and sabotage.

Iranian officials have said in recent months, however, that they plan to produce more of the fuel enriched to 20 percent purity than is needed for the reactor. “They have now produced nearly enough 20 percent to fuel the Tehran Research Reactor for the next 20 years,” one diplomat in Europe who closely follows the agency’s work in Iran said on Friday. The fact that Iran is increasing production further has heighted suspicions in the West that it wants to stockpile the fuel in case it decides, in the future, to produce bomb-grade material. It would take relatively little additional work to get that fuel to the 90 percent purity needed for weapon fuel.

Iranian officials deny that this is their intent, and Ayatollah Ali Khamenei, the country’s supreme leader, explicitly ruled out producing a weapon in a recent speech.

David E. Sanger reported from Washington, and William J. Broad from New York. (Copyright 2012 The New York Times Company 02/25/12)