Israel claims that in the last few days it has taken complete control of Iran's airspace, and that it has set back the country's nuclear program immensely with targeted attacks on key sites and people.

So why is Prime Minister Benjamin Netanyahu still courting a US entry into the conflict?

It comes down to one weapon.

Iran's key nuclear enrichment facility, Fordow, is buried up to 100m below the surface of the ground, rendering it all but impervious to Israeli aerial action.

And observers say that while this facility is online, Iran's nuclear program may be set back, but it's not knocked out.

"There is only one weapon in the world that can destroy a place like Fordow, and there's only one plane in the world that can carry that weapon," retired US Air Force colonel and CNN military analyst Cedric Leighton said.

That weapon is the "bunker buster" that has been brought up at times during the ongoing turmoil.

More properly, it's the GBU-57, a massive ordnance penetrator (MOP).

The MOP is a six-metre, 13.6-tonne bomb that can strike a deeply buried target after multiple impacts with "pinpoint accuracy", Leighton said.

And the only plane in the world capable of carrying it is the USAF B2 bomber.

"Israel doesn't have any of these planes, or any of these bombs, and that's why they want the US to join their aerial campaign against Iran," Leighton said.

President Donald Trump has given himself a two-week deadline to decide whether the US will do so.

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