U.N. nuke agency to visit Tehran on Jan. 28

U.S. Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying and the U.S administration condemned the attacks. Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement.

Beyond urging Iranian cooperation with the IAEA probe of the alleged weapons work, the U.S. and its allies are pressuring Iran to halt uranium enrichment, a key element of the nuclear program that dozens of nations suspect is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel, but at higher levels it can be used as material for a nuclear warhead.

Iran denies it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity.

Those claims were called into question on Monday when the IAEA confirmed Iran had begun increasing its production of uranium enriched to 20 percent. That’s a significantly higher concentration than the nation’s main stockpile — and can be turned into weapons-grade material more quickly than the lower enriched uranium.

Olli Heinonen, Nackaert’s predecessor, noted that “if Iran decides to produce weapons-grade uranium from 20 percent enriched uranium, it has already technically undertaken 90 percent of the enrichment effort required.”

“This does not automatically mean Iran will be able to build a nuclear weapon in one month — building an atomic bomb is a complex endeavor that requires precision engineering capabilities that Iran may lack,” wrote Heinonen, in a commentary for Foreign Policy magazine. “But it does mean that the country would be able to ‘break out’ of its international obligations very quickly should it decide to do so.”

• Michael Astor contributed to this report from the United Nations.

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