"We have degraded their program by one to two years at least -- intel assessments inside the (Defense) Department assess that," Pentagon spokesman Sean Parnell told journalists, later adding: "We're thinking probably closer to two years."

American B-2 bombers hit two Iranian nuclear sites with massive GBU-57 bunker-buster bombs last month, while a guided missile submarine struck a third site with Tomahawk cruise missiles.

Israel launched an unprecedented air campaign targeting Iranian nuclear sites, scientists and top military brass on June 13 in a bid to end the country's nuclear program, which Tehran says is for civilian purposes but Washington and other powers insist is aimed at acquiring atomic weapons.

US President Donald Trump had spent weeks pursuing a diplomatic path to replace the nuclear deal with Tehran that he tore up during his first term in 2018, but he ultimately decided to take military action.

The US operation was massive, involving more than 125 US aircraft including stealth bombers, fighters and aerial refueling tankers as well as a guided missile submarine.