“We don’t know where [the Iranians] are today,” said Olli Heinonen, the former deputy director of the International Atomic Energy Agency (IAEA), whose inspectors have long been tasked by the U.N. Security Council with determining the status of Iran’s disputed nuclear program.

Mr. Heinonen, who spent 27 years at the IAEA before his current role as a senior fellow at Harvard University’s Kennedy School of Government, told reporters Tuesday IAEA inspectors cannot be certain Iran is not hiding thousands of advanced IR-2m uranium enrichment centrifuges.

There is a possibility, he said, that the Islamic Republic is in posession of thousands more of the centrifuges than it has declared during talks with other world powers — a situation that could mean Tehran is actually on the verge of developing a nuclear weapon.

“If you have 1,000 of these centrifuges and you start with natural uranium, at the end of one year, you have enough material for one nuclear device at least,” Mr. Heinonen said. “If you take 2,000 of these centrifuges and natural uranium, it will be half [the time]. If you take 4,000 of them, it will be three months.”

But the time it takes to reach “breakout capacity” for a nuclear weapon is cut “to less than half,” Mr. Heinonen said, if one begins with low-enriched uranium — something Iran has sought the right to possess during ongoing talks with the U.S. and other world powers.

His comments, during a conference call with reporters, came just days after U.S. and Iranian officials said they had failed to make progress toward a comprehensive deal that would curb Iran’s nuclear program ahead of a much anticipated Nov. 24 deadline for reaching such an agreement.

The remarks also follow news that the IAEA’s current leadership had voiced frustration in a recent classified report about Iran’s unwillingness to provide international inspectors with access to its nuclear sites.

“Iran has not provided any explanations that enable the agency to clarify” whether Tehran’s nuclear activities may be aimed at weapon-making, stated the confidential document, which was obtained by the Associated Press.

The bottom line, according to Mr. Heinonen on Tuesday, is that the IAEA has simply not been able fully examine Iran’s uranium and centrifuge stocks during the past year.

Tehran had promised to allow such an examination last November, when Iranian negotiators entered into an initial framework agreement and “joint plan of action” with the so-called P5+1 nuclear negotiating group, the five permanent members of the U.N. Security Council — the U.S., Britain, France, Russia and China, plus Germany.

Western powers have for years suspected Iran is clandestinely pursuing nuclear weapons. Tehran denies the charge, claiming its nuclear program is for purely peaceful purposes.

Mr. Heinonen said Tuesday that with a year now gone by, the IAEA knows some of what Iran has in stock, but simply does not have “a complete picture.”

“This is because the IAEA’s access to this ‘joint plan’ actually has been very limited,” he said.