Iran is poised to greatly expand uranium enrichment at a fortified underground bunker to a point that would boost how quickly it could make nuclear warheads, diplomats said.
They said Tehran had put finishing touches for the installation of thousands of new-generation centrifuges at the cavernous bunker - machines that can produce enriched uranium much more quickly and efficiently than its present machines.
While saying that the electrical circuitry, piping and supporting equipment for the new centrifuges was now in place, the diplomats emphasised that Tehran had not started installing the new machines at its Fordo facility and could not say whether it was planning to.
Still, the senior diplomats suggested that Tehran would have little reason to prepare the ground for the better centrifuges unless it planned to operate them.
The reported work at Fordo appeared to reflect Iran's determination to forge ahead with nuclear activity that could be used to make atomic arms despite rapidly escalating international sanctions and the latent threat of an Israeli military strike on its nuclear facilities.
Fordo could be used to make fissile warhead material even without such an upgrade, the diplomats said.
They said that although older than Iran's new generation machines, the centrifuges now operating there can be reconfigured within days to make such material because they already are enriching to 20% - a level that can be boosted quickly to weapons-grade quality.
In contrast, Iran's older enrichment site at Natanz is producing uranium at 3.4%, a level normally used to power reactors. While that too could be turned into weapons-grade uranium, reassembling from low to weapons-grade production is complex, and retooling the thousands of centrifuges at Natanz would probably take weeks.
The diplomats' comments come as International Atomic Energy Agency inspectors are due to visit Tehran. Their trip - the second this month - is another attempt to break more than three years of Iranian stonewalling about claims that it has - or is - secretly working on nuclear weapons that would be armed with uranium enriched to 90% or more.
Iranian officials deny nuclear weapons aspirations, saying the claims are based on bogus intelligence from the US and Israel. But IAEA chief Yukiya Amano has said there are increasing indications of such activity.