Satellite image of Iran's Fordo uranium enrichment facility By Forrest Crellin
The tunnel complex is the only target that appears not to have been badly damaged in attacks last June by Israel and the U.S. on Iran's nuclear facilities.
Diplomats have long said Isfahan has been used to store 60% uranium, which the International Atomic Energy Agency confirmed in a report to member states last month, without saying how much was there.
IRAN STILL HAS HIGHLY ENRICHED URANIUM STOCKS
The IAEA estimates that when Israel launched its first attacks in June, Iran had 440.9 kg of 60% uranium. If enriched further, that would provide the explosive needed for 10 nuclear weapons, according to an IAEA yardstick.
"What we believe is that Isfahan had until our last inspection a bit more than 200 kg, maybe a little bit more than that, of 60% uranium," IAEA chief Rafael Grossi told reporters in Paris.
He said the stock was "mainly" at Isfahan, and some held elsewhere may have been destroyed.
"The widespread assumption is that the material is still there. So we haven't seen - and not only us, I think in general all those observing the facility through satellite imagery and other means to see what's going on there - movement indicating that the material could have been transferred," Grossi said.
Iran has not informed the IAEA of the status or whereabouts of its highly enriched uranium since the June attacks, nor has it let IAEA inspectors return to its bombed facilities.
Iran's nuclear programme is one reason Israel and the U.S. have given for their current attacks on Iran, arguing that it was getting too close to being able to produce a bomb, despite Trump saying in June that U.S. strikes had obliterated the programme. The IAEA has said it has no credible indication of a coordinated nuclear weapons programme.
All three Iranian uranium-enrichment plants known to have been operating - two at Natanz and one at Fordow - were destroyed or badly damaged in June.
"There is an amount (of 60% uranium) in Natanz also, which we believe is still there," Grossi said.
(Writing by Francois Murphy; Editing by Sharon Singleton and Kevin Liffey)