WASHINGTON (NYTIMES) - Israeli and United States intelligence officials have been watching each day as Iran digs a vast tunnel network just south of the Natanz nuclear production site, in what they believe is Teheran's biggest effort yet to construct new nuclear facilities so deep in the mountains that they can withstand bunker-busting bombs and cyberattacks.

Although the construction is evident on satellite photographs and has been monitored by groups that track the proliferation of new nuclear facilities, Biden administration officials have never talked about it in public and Israel's defence minister has mentioned it just once, in a single sentence in a speech last month.

In interviews with national security officials in both nations, there clearly were differing interpretations of exactly how the Iranians may intend to use the site, and even how urgent a threat it poses.

But as President Joe Biden prepares for his first trip as president to the Middle East next month - one that will take him to Israel and Saudi Arabia, Iran's two biggest regional rivals - there is little debate that the conflict over Iran's nuclear programme is about to flare again.

By most accounts, Iran is closer to being able to produce a bomb today than at any other point in the two-decade-long saga of its nuclear programme - even if it is planning, as many national security officials believe, to stop just short of producing an actual weapon.

On Mr Biden's trip, the question of taking more extreme measures to stop Iran, as the United States and Israel have attempted before, will be high on the agenda.

The International Atomic Energy Agency (IAEA) said this month that the country is just weeks away from being able to enrich enough bomb-grade fuel to make a single nuclear bomb - although fashioning that into a usable weapon could take at least another two years, even by the most alarmist Israeli estimates.

General Kenneth McKenzie Jr, who retired recently as the head of US Central Command, where he oversaw military planning for dealing with Iran, said Teheran, at least in the short term, was trying to leverage its nuclear capabilities as it negotiates with the US.

"The Iranians' highest priority is using the nuclear threat to gain concessions, economic and otherwise," he said.

But the facility could eventually prove critical to Iran if the Biden administration's efforts to revive the 2015 nuclear continues to run into roadblocks. And for now, at least, efforts to reimpose limits on Iran's nuclear actions appear all but dead.

The deal, which then-president Donald Trump abandoned in 2018, limited Teheran's ability to install new centrifuges and forced it to ship 97 per cent of its nuclear fuel out of the country.

Mr Biden's refusal of Iran's demand to remove the Revolutionary Guard from the list of terrorist organisations, along with a flow of new revenue to Teheran resulting from today's soaring oil prices, have contributed to the stalemate in the talks.

Now, the Iranians are looking for new pressure points, including the excavation of the mountain plant near Natanz. And over the past week, Iranian authorities have switched off 27 cameras that gave inspectors a view into Iran's production of fuel.

The decision to cut off the cameras, which were installed as part of the nuclear deal, was particularly worrisome to Mr Rafael Grossi, director-general of the IAEA, the United Nations agency responsible for nuclear inspections.

If the cameras remain off for weeks and it is impossible to track the whereabouts of nuclear materials, "I think this would be a fatal blow" to hopes of reviving the accord, Mr Grossi said last week.

For decades, a barren piece of land near Natanz has been the centrepiece of Iran's nuclear effort. The country has always insisted that its underground "pilot plant" there is working only to produce nuclear fuel for peaceful purposes - the production of nuclear energy.

The evidence, some of it stolen by Israel from a warehouse in Teheran, suggests otherwise: that Iran has had plans in place for two decades to construct a bomb, if it concluded that it was in its interest.

Although the pilot plant is underground, it is not deeply buried, making it an easy target for a bombing attack - a step Israel has come close to taking on many occasions.

But the largest attack on the Natanz site came not from the air but from a combined US-Israeli cyberoperation, code named "Olympic Games'', which forced the plant's nuclear centrifuges, which rotate at supersonic speeds, to spin out of control.

During the Bush and Obama administrations, it destroyed hundreds of centrifuges, and set Iran's programme back by a year or more. But it was no silver bullet.

To protect its future programmes, Iran began building facilities deep underground. Its biggest success so far is a site called Fordow, built under a mountain on a base run by the Revolutionary Guard. It was exposed in September 2009, early in the Obama administration, when Iran, realising that the cover had been blown on its project, hurriedly told the IAEA of its existence before then-president Barack Obama and then-French president Nicolas Sarkozy could announce the finding.

The new facility is close to Natanz, but it resembles Fordow, which would require the largest bunker-busting bombs in the US inventory to attack. Israel does not yet possess those bunker-busters or have a means to deliver them.

Biden administration officials say they have been following the construction of the new facility for more than a year but are not especially alarmed. It is still several years from completion. And they suspect its immediate purpose is to replace a centrifuge assembly facility that Israel blew up in April 2020, in a particularly sophisticated attack that made clear that the Iranian programme had been penetrated by insiders, who apparently planted the explosives.

Several Israeli officials say they believe Iran's ultimate objective is to use the facility to enrich uranium at a mass scale, using a family of advanced centrifuges that Iran has already started installing, on a test basis, at its older facility nearby. US officials concede the new facility is quite large and usually well protected.

Still, the US is not convinced that Iran intends to use the facility to enrich uranium. That remains a possibility, given its size, but it is not a certainty, senior US officials said. What is more clear, these officials said, is Iran's intention to use the facility to construct centrifuges, rebuilding facilities the Israelis have destroyed in recent years.