Iran appears to be building a nuclear facility so deep in the earth that it is probably beyond the range of a last-ditch US weapon designed to destroy such sites.

The Associated Press came to the conclusion after speaking to experts and analysing satellite imagery.

Completing such a facility “would be a nightmare scenario that risks igniting a new escalatory spiral [over Iran’s nuclear programme],” Kelsey Davenport, director of non-proliferation policy at the Washington-based Arms Control Association, was cited as saying, adding: “Given how close Iran is to a bomb, it has very little room to ratchet up its programme without tripping US and Israeli red lines. So at this point, any further escalation increases the risk of conflict.”

Photos and videos from Planet Labs PBC show Iran – which can now produce uranium close to weapons-grade – has been digging tunnels in a mountain near the Natanz nuclear site amid the Zagros Mountains in central Iran.

Iran says the new facility will replace an above-ground centrifuge manufacturing centre at Natanz damaged by an explosion and fire in July 2020. Tehran pinned the incident on Israel, long suspected of running sabotage missions against Iran’s nuclear programme, despite the Iranians’ insistence they have no ambition to move it from the civilian sphere to military territory.

If the US and Israel came to the conclusion that Iran was pursuing the building of a bomb, would the new nuclear facility be immune from attack in the event of diplomacy failing?

Not according to Israeli National Security Adviser Tzachi Hanegbi.

Speaking to the Herzliya Conference, an international security forum in Jerusalem on May 23, Hanegbi, according to AP, said: "This of course limits the capacity to carry out an attack, relative to above-ground facilities, which is of course easier. But what can be said about this matter is that there is nowhere that cannot be reached.”

Hanegbi was also reported as declining to threaten an explicit Israeli attack and as suggesting the onus would be on the US, noting it has Massive Ordinance Penetrator bombs that are not in Israel's arsenal.

He also observed that the underground facility “is years away from being completed”.

Separately, Israel on May 23 accused Iran’s Islamic Revolutionary Guards Corps (IRGC) of turning commercial ships into platforms for launching missiles, drones and commandos.

Israeli Defence Minister Yoav Gallant was reported by Reuters as showing images of six purportedly repurposed Iranian vessels that were "floating terror bases".

The past two years have seen Iran and Israel trade blame over a series of unclaimed attacks on their ships in the Middle East.