Israeli forces have struck many of Iran’s nuclear sites, but will likely need U.S. assistance to destroy Iran’s most hardened facility.

The Fordow Fuel Enrichment Plant, located within the Kūh-e Dāgh Ghū’ī mountain, is northeast of the city of Qom and about a hundred miles south of Tehran. It is one of two underground enrichment facilities in Iran.

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Israel does not have the “bunker-busting” bombs believed to be required to destroy a facility so far underground, but the United States does.

“We know that Fordow, that is their facility that is a kilometer or half a kilometer beneath the ground, where they are doing a lot of enrichment. That’s the key, that’s the difference, Brian, between making this a technical knockout and absolute knockout. But that is going to be up to President [Donald] Trump and the American people,” Doron Spielman, an Israeli military spokesman, said on Fox News’s Brian Kilmeade Show on Monday.

The weapon the U.S. has but Israel doesn’t is the Massive Ordnance Penetrator. It’s a 30,000-pound bomb with a 6,000-pound warhead specifically designed to destroy hardened and buried facilities. It’s so large that the B-2 Spirit stealth bomber and B-21 Raider must carry it once it’s operational.

There is a “huge difference between American capabilities and [Israel’s], and the only ones who can destroy, for instance, this facility in Fordow, is the Americans,” Retired Maj. Gen. Giora Eiland, the former head of Israel’s national security council, acknowledged during an American Middle East Press Association briefing on Monday.

While Israel has targeted other facilities, nuclear scientists, military leaders, and Iran’s arsenal, its operation against Iran would be incomplete without destroying Fordow.

If the Israelis are unable to destroy the facility it could push the Iranians to “continue to pursue this program or even try to make a dash to take what they have now and try to create some kind of weapons,” retired Gen. Joseph Votel, former commander of U.S. Central Command, said on ABC’s This Week.

President Donald Trump and administration officials have repeatedly said the U.S. is not a part of the Israel-Iran war, and does not want to get bogged down by another war in the Middle East. It puts the onus on Trump, who has urged Iran to come back to the negotiating table to end the conflict, to decide whether the U.S. should get involved in the war and to what extent.

The U.S. military is aiding Israel to intercept Iranian-fired missiles headed for the Jewish State, but administration officials have distanced the U.S. from Israel’s offensive attacks against Iran.

One consideration for Trump is that Israeli officials say they have established air supremacy over western Iran.

Having air superiority in a conflict allows for one side to fly over the conflict area without significant fear of getting fired upon.

While Trump has publicly called himself the most pro-Israel president since Israel’s founding, he also campaigned last year on avoiding U.S. involvement in conflict across the globe, and has sought to negotiate the end to multiple wars.

In the days leading up to Israel’s initial salvo against Iran, Trump had publicly emphasized the U.S.-Tehran negotiations. Since the conflict began days ago, the president has pushed Iran to get back to the negotiating table, saying negotiators “can easily get a deal done.”

HOW ISRAEL CHANGED ITS IRAN CALCULATION

During the conflict’s first few days, Israel destroyed buildings above ground at the Natanz facility, but there are conflicting reports about the below-ground structures. Spielman said Israel had destroyed the underground facilities, which was contradicted in comments from Rafael Grossi, the International Atomic Energy Agency’s director-general.

Israel also targeted another nuclear facility in Isfahan. Grossi said on Monday that there was no sign of increased radiation at that location.