Iran has significantly stepped up uranium enrichment at a secret facility, the United Nations atomic watchdog has said, emphasising that Tehran needs to address “serious concerns” about “possible military dimensions” to its nuclear programme.

The latest report by experts of the International Atomic Energy Agency (IAEA) disclosed a 42 per cent rise in the number of operational centrifuges enriching uranium inside the Natanz facility, located in central Iran, in the last four months.

Work inside a second, previously secret enrichment plant has also been stepped up, with 698 centrifuges operating inside the Fordow installation near Qom, southwest of Tehran, a 69 per cent increase on the number recorded during the last inspection in October, the Daily Telegraph reported.

Iran had declined to offer full co-operation to its inspectors when they visited the country earlier this month, the U.N. atomic watchdog said on Friday.

In particular, they were prevented from visiting a military location at Parchin, where experiments that would only be relevant to mastering the detonation system of nuclear weapons are understood to have been conducted.

The watchdog bemoaned “major differences” with Iran after two fruitless visits, the paper said.

The latest report by the IAEA is likely to deepen Israel’s fears about Iran’s intentions, particularly the build-up of centrifuges in the Fordow facility, which is dug into a mountainside and could be immune from military attack.

The IAEA reported that 8,808 centrifuges were functioning inside Natanz, compared with 6,208 on its last visit, although the report cautioned that not all of the machines may have been working. Iran’s experts had also installed the casings for another 6,177 centrifuges, it said.

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