WASHINGTON — The United Nations' nuclear watchdog said Thursday it suspects that Iran might be trying to develop a nuclear warhead that could be placed atop a missile, its sharpest challenge to date of Iran's claims to be pursuing an exclusively peaceful nuclear program.
The report by the International Atomic Energy Agency conflicts with a 2007 U.S. intelligence community assessment that Iran halted its nuclear weapons program in the fall of 2003. It is more in line with reported European and Israeli estimates that Iran, along with enriching uranium that could fuel a nuclear bomb, is experimenting with constructing a warhead.
The Vienna-based IAEA said it had collected "broadly consistent and credible information" from "a variety of sources" about Iran's military-related nuclear activities. "Altogether, this raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile," it said.
Senior Obama administration officials called the report disturbing, and said the U.S. assessment of Iran's nuclear ambitions is under review as part of a new National Intelligence Estimate. The officials briefed reporters on the condition of anonymity under White House-imposed ground rules.
The IAEA report also confirms that Iran, as it claimed last week, has begun enriching uranium to a purity of nearly 20 percent, closer to what's needed for nuclear bomb fuel.
A U.S. official, however, said Iran is producing an estimated 100 grams per day of the 20-percent pure uranium — at a rate that would take it five to seven years to have enough for a bomb if it were enriched further. Weapons-grade uranium is roughly 90 percent pure.
Perhaps more worrisome, the IAEA said Iran has moved most of its current stock of low-enriched uranium to a pilot enrichment plant, indicating it is planning to convert it to the more-pure form.
The steps Iran is suspected of taking toward a nuclear warhead have been cited in previous IAEA and media reports.
The agency's overall conclusions, however, suggest a hardening stance from the IAEA in dealing with Iran under its new director general, Japanese diplomat Yukiya Amano.
"Amano approaches his job with a different philosophy," avoiding politics and focusing on unanswered questions about Iran's activities, one U.S. official said.
The report also seems likely to impact high-stakes diplomacy over new sanctions on Iran. China is resisting a U.S.-led push for new action by the U.N. Security Council.
U.S. officials and private analysts cautioned that the document also shows that Iran is continuing to encounter technical difficulties in its nuclear program.
It has 3,772 centrifuges running at its plant in Natanz, the IAEA said, fewer than in November. That could indicate Iran hasn't overcome problems with its centrifuges, rapidly-spinning machines that enrich uranium.
Leonard Spector, a former Energy Department official, said the new enrichment could be mostly "a demonstration for the sake of thumbing their nose at the international community." Still, he said, "They've just taken another not-so-small step up the ladder" of nuclear capabilities.
Iran denies it's seeking nuclear weapons, and says it needs the purer uranium to fuel a Tehran research reactor that produces nuclear isotopes for medical uses, such as cancer treatment.
An Iranian official said Thursday that the report confirmed its position. "The IAEA's new report confirmed Iran's peaceful nuclear activities and the country's non-deviation towards military purposes," Tehran's envoy to the agency, Ali Asghar Soltanieh, told Iran's state-run media.
The U.S. intelligence community's 2007 conclusion that Iran was no longer actively working on a nuclear weapon was controversial from the moment it was issued, and disputed by Israel, among others.
In congressional testimony earlier this month, Director of National Intelligence Dennis Blair said, "We continue to assess Iran is keeping open the option to develop nuclear weapons in part by developing various capabilities that bring it closer to being able to produce such weapons . . . We do not know, however, if Iran will eventually decided to build nuclear weapons."
The 10-page IAEA report said Iran's alleged weapons work "seems to have continued beyond 2004," contradicting the 2007 U.S. intelligence estimate.
The IAEA didn't disclose its sources on Iran's suspected weapons programs, but they're thought to be U.S., European and Israeli intelligence agencies. A key piece of evidence is a laptop belonging to an Iranian nuclear scientist that was smuggled out of the country in 2004. Iran has said the laptop documents are fabricated.
Iran has declined to discuss the issue with IAEA officials since August 2008, it said.
The agency said its questions include whether Iran tried to design a new missile chamber capable of carrying a nuclear warhead; whether it experiments with high-precision detonators "were solely for civil or conventional military purposes"; and whether it developed a spherical implosion system "possibly with the assistance of a foreign expert."
When Iran announced last week that it had enriched uranium to 20 percent, U.S. and European officials questioned the claim. French Foreign Minister Bernard Kouchner dismissed it, and White House spokesman Robert Gibbs said it was "based on politics, not on physics."
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