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 as 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. "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."

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 an 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.

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