July 15 (Reuters) - U.S. President Donald Trump threatened to attack a site linked to Iran's nuclear program known as Axe Mountain, a fortified facility buried deep underground near one of Iran's main nuclear sites.

"We will destroy Axe Mountain," Trump said in an interview on July 13. "Tell the Iranians to get ready."

This threat reflects escalating tensions amid the exchange of strikes between Tehran and Washington in the Gulf, hindering efforts to end the conflict.

Here's what we know about "Axe Mountain":

Where is it located?

Mount Faas is located 220 kilometers south of Tehran and two kilometers from the Natanz nuclear complex.

The Natanz site, where two of Iran’s uranium enrichment plants are located, was bombed during the war launched by the United States and Israel on February 28, and during the 12-day war last year.

The Institute for Science and International Security, a US-based research center focused on preventing the spread of nuclear weapons, said the facility, which is still under construction at Axe Mountain, was not targeted in either of these wars.

The mountain's summit rises to about 1600 meters above sea level.

There were two enrichment facilities operating at Natanz, one above ground and one underground. The UN's International Atomic Energy Agency (IAEA) reported that the above-ground facility was destroyed. The underground facility was likely at least severely damaged.

What is the history of this site?

The site is linked to Iran’s nuclear program, which has long strained relations between the West and Iran, which denies seeking to acquire an atomic bomb.

The institute said construction of the facility at Jabal al-Fas began in 2020 following what Iranian authorities at the time said was an explosion caused by sabotage at the Natanz facility.

Iran said at the time that the sabotage operation at Natanz had caused serious damage that could slow the development of advanced centrifuges for uranium enrichment.

In September of that year, the then-head of Iran’s Atomic Energy Organization, Ali Akbar Salehi, said that Iran had begun building “a more modern, larger, and more comprehensive hall in all respects in the heart of the mountain near Natanz” to manufacture advanced centrifuges.

Rafael Grossi, the director general of the UN's International Atomic Energy Agency, noted in an interview with PBS Frontline in March that Iran had previously announced its intention to conduct nuclear activities at Jabal al-Fas.

He said, "This was part of their entirely systematic intention to put their most sensitive facilities underground."

What did Iran build there?

The institute, which analyzed satellite images of the site, says it has two entrances that are supposed to lead to a single facility estimated to be at least 100 meters below the mountain.

The institute stated in a report issued on July 14 that the physical defense measures consist mainly of a large-scale security perimeter and extensive reinforcements for tunnel entrances.

The institute's report noted that the two eastern entrances to the tunnel had been partially filled in since the two wars to impede the arrival of land vehicles, but they were not completely closed.

Sam Lear, a researcher at the Foreign Policy Research Institute who also reviewed recent satellite images of the site, told Reuters that strengthening the entrances would complicate "targeting them with bunker-buster munitions such as bunker-buster bombs."

Is the website operational and what purpose can it be used for?

Trump said in his remarks on July 13 that Washington is closely monitoring Mount Axe.

"We don't see any activity there," he said. "They're not doing very well with regard to their nuclear status. Every time we hear about it, we blow it up. So they don't like to talk about it. But we'll probably strike Mount Axe relatively soon."

The institute said in its report that its assessment "indicates that the facility has not yet started operating, but construction work is ongoing," and that it is unclear when it could start operating based on satellite imagery alone.

He added, "It is also unclear whether Iran still plans to establish a large-scale assembly facility, given the destruction of Iran's centrifuge program, including Iran's ability to manufacture the centrifuge components needed for the assembly plant."

He continued, "However, if Iran begins to rebuild its centrifuge manufacturing capacity, it may plan to establish a smaller centrifuge assembly facility at Jabal al-Fas that could serve a nuclear weapons program."

How can the site be attacked?

Experts believe that the complex, located at a great depth, is outside the range of the most powerful bunker-buster bombs in the US arsenal.

The institute said the site "would be more conducive to attack or sabotage by ground forces."

He added, "However, there may be weaknesses that can be exploited by deep-penetrating weapons via air attacks."

"We can conclude that there are ongoing activities at Jabal al-Fas that the Iranians would like to continue, but they are still concerned enough about the possibility of an attack that they are taking steps to strengthen their defenses," Lear said.