Iran is blocking U.N. nuclear-agency attempts to upgrade monitoring of its atomic program while advancing those activities to the stage that the country would have the means to test a weapon within six months, diplomats said Friday.

VIENNA — Iran is blocking U.N. nuclear-agency attempts to upgrade monitoring of its atomic program while advancing those activities to the stage that the country would have the means to test a weapon within six months, diplomats said Friday.

The diplomats emphasized that there were no indications of plans for such a nuclear test, saying it was highly unlikely Iran would risk heightened confrontation with the West — and chances of Israeli attack — by embarking on such a course.

But they said that even as Iran expands uranium enrichment, which can create fissile nuclear material, it is resisting International Atomic Energy Agency attempts to increase surveillance of its enrichment site meant to keep pace with the plant’s increased size and complexity.

For Iran to amass enough fissile material to conduct an underground test similar to North Korea’s 2006 nuclear explosion, it would likely have to kick out monitors of the IAEA — the U.N. nuclear agency — from its one known uranium-enrichment site at Natanz. Technicians then could reconfigure the centrifuges now churning out nuclear-fuel-grade enriched uranium to highly enriched, weapons-grade material.

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Iran is unlikely, however, to want to do that. Such a move would immediately set off international alarm bells and could bridge rifts on how strongly to react — Russia and China, which have resisted Western calls to increase pressure on Iran over its nuclear defiance, would likely endorse more sweeping U.N sanctions and other penalties.

With the U.N. nuclear agency strictly limited in its nuclear monitoring of Iran, the existence of a hidden enrichment site that could supply the weapons-grade uranium needed for a nuclear weapons test is also possible.

International Atomic Energy Agency chief Mohamed ElBaradei has repeatedly warned that his agency cannot guarantee that Iran is not hiding nuclear activities. Iranian nuclear expert David Albright on Friday put the chances that such a secret site exists at “50-50.”

But even a hidden enrichment plant meant to upgrade material to weapons level would likely have to be fed with low-enriched uranium from the Natanz site. So transporting that material would not escape the agency’s detection.

In any case, international action — and possible Israeli attack — would be triggered at the latest by a nuclear test explosion.

Iran is still considered years away from developing a reliable nuclear-warhead delivery system. So tipping its hand with a nuclear test, should it want to own such weapons, would make little sense.

“We are talking here not of intent but capability,” said one of two western diplomats accredited to the International Atomic Energy Agency. Like his colleague from another country, this diplomat — who has access to intelligence on Iran’s nuclear program — demanded anonymity in exchange for discussions of the highly confidential issue.