BY DAVID E. SANGER AND WILLIAM J. BROAD
New York Times

The director of the U.N. nuclear watchdog declared in unusually blunt language on Thursday that Iran has stonewalled investigators about evidence that the country had worked on nuclear weapons design, and that his efforts to reveal the truth had “effectively reached a dead end.”

The comments by the official, Mohamed ElBaradei, came four days before he leaves office after 12 years at the head of the International Atomic Energy Agency. His remarks refocused attention on Iran’s alleged work on weapons design at the moment that the West is considering moving to harsher economic sanctions on Tehran, after it backed away from a commitment it made in early October to temporarily ship much of its nuclear fuel out of the country.

ElBaradei’s remarks also came as Iran approaches President Barack Obama’s end-of-year deadline to reassess whether the United States should move toward what Secretary of State Hillary Rodham Clinton has termed “crippling sanctions” on Iran. Israeli officials, meanwhile, have said they would not consider taking military action until Obama’s deadline runs out, leaving hanging the suggestion — maybe the bluff _ that it was preparing for that possibility in 2010.

ElBaradei’s statement marked a sharp departure in tone, and a tacit acknowledgment that his behind-the-scenes effort to broker a deal have collapsed. In the past, he has privately talked about Iran’s refusal to answer the agency’s questions about weapons work, but has stopped short of rebuking the country in public for fear of shutting off any chance of future cooperation.

Those questions, posed by the agency over a period of years, go to the heart of suspicions that Iran has worked on nuclear weapons designs. Among them are queries about drawings, computer simulations and other evidence of work that could not plausibly be involved in civilian nuclear power programs. That includes documents obtained by the agency _ some provided by Western intelligence services, who say they were slipped out of Iran by scientists _ that appear to show that Iran worked on how to shape uranium into the kind of hemispheres used for nuclear cores, on conventional explosions needed to detonate a nuclear chain reaction, and simulations of a warhead detonation at about 2,000 feet, about the height at which the bomb was set off over Hiroshima in 1945.

“It is now well over a year since the agency was last able to engage Iran in discussions about these outstanding issues,” ElBaradei said in remarks to the nuclear agency’s governors. “We have effectively reached a dead end, unless Iran engages fully with us.”

In the past, Iran has called the evidence “fabrications.” ElBaradei has complained that he has been prohibited by “member states,” including the United States and European nations, from letting the Iranians see the original evidence _ presumably for fear that it could reveal sources. On Thursday, he repeated his frustration on that point, telling the agency’s 35-member board that “it would help if we were able to share with Iran more of the material that is at the center of these concerns.”

It is unclear whether ElBaradei’s comments will help push Russia and China to vote in favor of a resolution condemning Iran for failing to tell the agency, until two months ago, about a uranium enrichment plant that it secretly built on a Iranian Revolutionary Guard base near the city of Qum. Iran later said that it kept the construction secret until recently because it feared that its known nuclear plants could be bombed.

But that delay violated its obligations to the United Nations, ElBaradei said, and his statements reinforced the sense that Iran has blocked inspectors from getting near what are known as “Project 110″ and “Project 111,” its suspected weapons-design work.

At Iran’s invitation, however, inspectors visited the underground plant at Qum last month, and confirmed it is in the final stages of construction, but not yet operational. It is supposed to house 3,000 centrifuges _ too small, experts say, to be useful for producing civilian nuclear fuel, but large enough to produce about two weapons’ worth of material each year.

American officials tried to use Iran’s concealment of that plant, and the possibility that there were related facilities built to produce nuclear material, to press Prime Minister Vladimir V. Putin of Russia and President Hu Jintao of China to join a new round of sanctions. Both have been reluctant, especially Hu, who said nothing about it during Obama’s trip to Beijing this month. Privately, American officials visiting China have told officials to weigh how a confrontation with Iran could interfere with China’s purchases of oil. China gets roughly 15 percent of its oil from Iran.

But the central issue through the Iran investigations has been the evidence suggesting that Iran conducted some level of research on weapons. An American intelligence estimate, published two years ago, contended that Iran ceased that work in 2003; intelligence agencies in Britain, France, Germany and Israel, examining the same evidence, have assessed that the work has resumed, or never stopped.

In October, parts of a confidential analysis written by senior staff members of the IAEA were leaked. The analysis concluded that Iran has acquired “sufficient information to be able to design and produce a workable” atom bomb. The report’s conclusions went beyond ElBaradei’s public positions, and even those taken by the United States and several governments.

The analysis drew a picture of a complex program, run by Iran’s Ministry of Defense, “aimed at the development of a nuclear payload to be delivered using the Shahab 3 missile system,” Iran’s medium-range missile, which can strike the Middle East and parts of southern Europe.

That analysis, and others like it, draw on years of clues and scraps of information gathered in Iran and from intelligence agencies around the world. For instance, atomic inspectors have found signs that Iran has done extensive research on high-voltage detonators, explosive lenses for bomb detonation, and re-entry vehicles for missiles that can cushion nuclear warheads as they streak earthward.

The inspectors also found evidence that a Russian scientist had helped Iran conduct complex experiments on how to detonate a nuclear weapon. They believe he acted on his own as an adviser on experiments described in a lengthy document that the agency obtained. Officials have described the original, in Farsi, as a detailed narrative of experiments aimed at achieving the perfectly timed compression of nuclear fuel to order to squeeze it into supercritical mass, which initiates a nuclear blast.

In 2006, the agency released a report saying that Iran had obtained from the global black market a document “related to the fabrication of nuclear weapon components.” The previous year it told of the market offering to help Iran shape uranium metal into “hemispherical forms,” which Western nuclear experts say are needed to make nuclear bomb cores.

Also in 2005, European and American officials told of an Iranian laptop computer that held studies for crucial features of a nuclear warhead, including a telltale sphere of detonators to trigger an atomic explosion. The documents specified a blast roughly 2,000 feet above a target _ considered high enough for a nuclear detonation to maximize the damage below.