A powerful US bunker-busting bomb was used in combat for the first time when Washington struck Iranian nuclear sites Sunday.

Only the US has the GBU-57 -- a 30,000-pound (13,600kg) weapon viewed as necessary to reach the most deeply buried facilities -- or the aircraft needed to deploy it.

General Dan Caine, the top US military officer, told journalists on Sunday that Washington's forces dropped 14 of the bombs in the massive operation aimed at knocking out Tehran's nuclear program.

The bombs are designed to penetrate up to 200 feet underground before exploding.

"To defeat these deeply buried targets, these weapons need to be designed with rather thick casings of steel, hardened steel, to sort of punch through these layers of rock," said another expert.

The 6.6-meter-long GBU-57 also has a specialized fuse as "you need an explosive that's not going to immediately explode under that much shock and pressure."

Caine said "initial battle damage assessments indicate that all three sites sustained extremely severe damage and destruction."

The only aircraft capable of deploying the GBU-57 is the B-2 Spirit, a stealth bomber.

The US employed seven B-2s in the Iran strikes -- aircraft that can fly 9,600km without refueling and which are designed to "penetrate an enemy's most sophisticated defenses and threaten its most valued, and heavily defended, targets," according to the US military.

"This was the largest B-2 operational strike in US history and the second-longest B-2 mission ever flown," Caine said.

Several B-2s proceeded west over the Pacific as a decoy while the bombers that would take part in the strikes headed east -- a "deception effort known only to an extremely small number of planners and key leaders," the general said.