A U.S. attack on Iran's key underground nuclear facility would not be a guaranteed success, according to a national security expert.

Fordo is Iran’s second nuclear enrichment facility after Natanz, its main facility, which already has been targeted by Israeli airstrikes. The IAEA said Tuesday it believes the strikes have had “direct impacts” on the facility’s underground centrifuge halls.

Donald Trump is weighing up whether to join Israeli strikes on Iranian nuclear facilities, with air and naval assets deploying to the Middle East. If the US President does give a green light, one target near-certain to be in the crosshairs is the Fordo uranium enrichment plant,