EXPLAINER: What is the GBU-57 and how can it be used against Iran's nuclear facility?

Israel's command of Iranian air space leaves few obstacles in the way of its expanding bombardment, though it will struggle to deal a knock-out blow to deeply buried nuclear sites without the U.S. joining the attack, experts say.

While Iran has responded with deadly missile strikes on Israeli cities, Israel has shown its military and intelligence pre-eminence as its warplanes have crisscrossed the Middle East, hitting Iranian nuclear installations, missile stockpiles, scientists, and generals - among other targets.

But even as Israeli officials laud the damage done so far, some publicly acknowledge Israel won't be able to completely knock out Iran's nuclear program - unless the U.S. joins the campaign with strategic bombers that can drop ordnance with the potential to penetrate sites buried deep underground.

B-2 BOMBERS AND 'BUNKER BUSTING' BOMBS

As many as six B-2 bombers relocated in March to a U.S.-British military base on the Indian Ocean island of Diego Garcia, U.S. officials have told Reuters, amid a U.S. bombing campaign in Yemen and mounting tensions with Iran.

There are only 20 B-2 bombers in the Air Force's inventory so they are usually used sparingly.

Experts say that puts the B-2s, which have stealth technology and are equipped to carry the heaviest U.S. bombs and nuclear weapons, in an ideal position to operate in the Middle East.

B-2 is equipped to carry America's the 30,000-pound GBU-57 Massive Ordnance Penetrator, designed to destroy targets deep underground. That is the weapon that experts say could be used to strike Iran's nuclear program.

FORDOW UNDERGROUND FACILITY

Iran's most deeply buried enrichment installation, dug deep into a mountain, has suffered little or no visible damage, Grossi reiterated on Monday.

While Fordow has only about 2,000 centrifuges in operation, it produces the vast majority of Iran's uranium enriched to up to 60%, using roughly the same number of centrifuges as the PFEP did, because it feeds uranium refined to up to 20% into those cascades, compared to 5% at the PFEP.

Fordow therefore produced 166.6 kg of uranium enriched to up to 60% in the most recent quarter. According to an IAEA yardstick, that is enough in principle, if enriched further, for just under four nuclear bombs, compared to the PFEP's 19.2 kg, less than half a bomb's worth.

Western powers accuse Iran of having a clandestine agenda to develop nuclear weapons by enriching uranium to a high level of fissile purity, above what they say is justifiable for a civilian atomic energy program.

Tehran says its nuclear program is wholly for civilian energy purposes.

(Production: Vanessa Romeo)