The US military did not use bunker-buster bombs on one of Iran’s largest nuclear sites last weekend because the site is so deep that the bombs likely would not have been effective, the US’ top general told senators during a briefing on Thursday.

The deep penetrating bombs that the U.S. dropped into two Iranian nuclear facilities were designed specifically for those sites and were the result of more than 15 years of intelligence and weapons design work.

U.S. strikes on Iranian nuclear sites inflicted "serious and significant damage," Iran has acknowledged for the first time since the attack nearly a week ago.