Iran has moved a stock of enriched uranium back underground after drawing what it needed to refine the material up to 20 percent purity, Tehran's envoy to the UN nuclear watchdog said on Monday.
Iran has said its move to feed low-enriched uranium (LEU) into centrifuges for higher-scale refinement is for peaceful purposes so that it can make fuel for nuclear medicine.
Western officials and UN inspectors doubt Iran's explanation since it lacks the technical capacity to convert higher-enriched uranium into fuel rods for its medical isotope reactor, whose Argentine-provided fuel stock is running out.
They fear Iran wants to advance along the road to producing high-enriched - 90 percent purity - uranium suitable for the fissile core of an atomic bomb, if it chose later to do so.
Iran said it started higher enrichment at the Natanz pilot nuclear fuel facility last month because it was frustrated at the collapse of an IAEA-backed plan for big powers to provide it with the necessary fuel rods.
Western diplomats also questioned why Iran had moved the bulk of its LEU - 1.95 tons - out of its underground main enrichment plant at Natanz, a much larger amount than would be needed to produce fuel for the reactor in the medium term.
"(This) was merely for producing material for the Iran research reactor. That is why that container is (now) back to its original location," Ali Asghar Soltanieh, Iran's ambassador to the International Atomic Energy Agency, told reporters.
He dismissed media reports that the move might have been a provocation aimed at adversaries such as Israel, which views the Iranian nuclear energy program as an existential threat.
The reports speculated that Iranian hardliners wanted to provoke an Israeli air strike on Iran's nuclear stockpile to provide a pretext for Tehran to kick out IAEA inspectors and develop nuclear weapons as a national security priority.
"For your information, (we) just moved the capsule because technically they needed it and they have put it back. We used we material which we needed for the Tehran Research Reactor," Soltanieh said, speaking in English during a break in a meeting of the IAEA's 35-nation board of governors meeting.
Diplomats there discounted the chance of political reasons for Iran having moved much of its LEU stockpile above ground.
"A more likely reason was that Iran needed a large container to provide a steady feed with sufficient pressure for 20 percent enrichment," said one senior diplomat close to the IAEA.
"In any case, this container can be moved back and forth between the pilot and main Natanz facilities in a half hour or so. So it could be moved quickly anyway," he told Reuters.