If the US decides to support Israel more directly in its attack on Iran, one option for Washington would be to provide the "bunker-buster" bombs believed necessary to significantly damage the Fordow nuclear fuel enrichment plant, built deeply into a mountain.
Such a bomb would have to be dropped from a US aircraft, which could have wide-ranging ramifications, including jeopardising any chance of Iran engaging in US President Donald Trump's desired talks on its nuclear program.
Israeli officials have also suggested that there are other options for it to attack Fordow as it seeks to destroy Iran's nuclear capabilities.
But Strategic Analysis Australia director Michael Shoebridge told the ABC that Israel would have difficulty destroying deep underground facilities in places such as Natanz and Fordow.
So what is a bunker-buster bomb and what type of plane can drop it?
What is a bunker-buster bomb?
"Bunker buster" is a broad term used to describe bombs that are designed to penetrate deep below the surface before exploding.
In this case, it refers to the latest GBU-57A/B Massive Ordnance Penetrator bomb in the American arsenal.
The roughly 13,600 kilogram precision-guided bomb is designed to attack deeply buried and hardened bunkers and tunnels, according to the US Air Force.
The bomb is believed to be able to penetrate about 60 metres below the surface before exploding, and the bombs can be dropped one after another, effectively drilling deeper and deeper with each successive blast.
Aside from a commando attack on the ground or a nuclear strike, the bunker-buster bomb seems the most likely option for causing meaningful damage against Fordow.
"The Israelis don't have the deep bunker-busting types of weapons," Mr Shoebridge said.
Israel only had access to a 2,270kg munition, he said, but would need the equivalent of an American 13,600 kilogram bomb.
In theory, the GBU-57A/B could be dropped by any bomber capable of carrying the weight, but at the moment the US has only configured and programmed its B-2 Spirit stealth bomber to deliver the bomb, according to the US Air Force.
The bomb carries a conventional warhead, but the International Atomic Energy Agency (IAEA) has confirmed that Iran is producing highly enriched uranium at Fordow, raising the possibility that nuclear material could be released into the area if the GBU-57A/B were used to hit the facility.
However, Israeli strikes at another Iranian nuclear site, Natanz, on a centrifuge site have caused contamination only at the site itself, not the surrounding area, the IAEA has said.
What is the Fordow nuclear site?
Fordow is Iran's second nuclear enrichment facility after Natanz, its main facility.
So far, Israeli strikes aren't known to have damaged Natanz's underground enrichment hall, nor have the Israelis targeted tunnels the Iranians are digging nearby.
Fordow is smaller than Natanz and is built into the side of a mountain near the city of Qom, about 95 kilometres south-west of Tehran.
Construction is believed to have started around 2006 and it became first operational in 2009 — the same year Tehran publicly acknowledged its existence.
In addition to being an estimated 80 metres under rock and soil, the site is reportedly protected by Iranian and Russian surface-to-air missile systems.
Those air defences, however, likely have already been struck in the Israeli campaign.
Israeli Prime Minister Benjamin Netanyahu has said the goal of attacking Iran was to eliminate its missile and nuclear program, which he described as an "existential threat" to Israel, and officials have said Fordow was part of that plan.
Israel's ambassador to the US, Yechiel Leiter, told Fox News last week that: "This entire operation … really has to be completed with the elimination of Fordow."
Chatham House international security research fellow Marion Messmer said Iranian officials had always maintained their nuclear program was for peaceful purposes only.
"We also know that Iran has been enriching uranium to a much higher degree than is necessary ... for that reason, there has been a long-running concern in the international community that Iran might be pursuing nuclear weapons," she said.
Still, Dr Messmer said: "We haven't really got strong evidence that Iran was trying to build a warhead or doing anything else prior to Israel's attack."
Would the US get involved?
Middle East analyst Rodger Shanahan told the ABC that the US had been moving military assets into the region to give Mr Trump "options" and "also to place pressure on the Iranian regime".
At the G7 meeting in Canada, Mr Trump was asked what it would take for Washington to become involved militarily and he said: "I don't want to talk about that."
In an interview over the weekend, Israeli ambassador Mr Leiter was asked about the possibility of the US helping attack Fordow and he emphasised Israel has only asked the US for defensive help.
"We have a number of contingencies … which will enable us to deal with Fordow," he said.
"Not everything is a matter of, you know, taking to the skies and bombing from afar."
Posting to social media on Tuesday (Washington time), Mr Trump called for Iran's "unconditional surrender".
"We now have complete and total control of the skies over Iran," he said.
"We don't want missiles shot at civilians, or American soldiers. Our patience is wearing thin."
How has Iran responded?
Iranian Deputy Foreign Minister Kazem Gharibabadi told state TV on Saturday that Iran would take measures to protect nuclear materials and equipment that would not be notified to the International Atomic Energy Agency and it would no longer cooperate with the IAEA as before.
Iranian lawmakers are also preparing a bill that could prompt Iran to pull out of the Non-Proliferation Treaty, following in the footsteps of North Korea, which announced its withdrawal in 2003 and went on to test nuclear weapons.
"The sticking point on the negotiations [with the US] to date has been whether Iran can conduct nuclear enrichment to civilian standards within Iran, or whether there is to be no enrichment whatsoever in Iran — and that's Washington's position at the moment," Dr Shanahan said.
The International Atomic Energy Agency does not know how many centrifuges Iran has outside its enrichment plants.
Any further reduction in cooperation with the IAEA could increase speculation that it will or has set up a secret enrichment plant using some of that supply.
ABC/wires
