How did US carry out strikes on Iranian nuclear sites?

US B-2 bombers and Washington's “bunker-buster” bombs were involved in the attacks on Iran's nuclear sites on Sunday, US officials have said.

Those bunker-busters were widely seen as the best chance of damaging or destroying the Fordow nuclear enrichment site, as it is deep underground. Israel's military is believed not to possess weapons capable of disabling the facility.

The US is also the only military capable of dropping the bombs. The Pentagon revealed on Sunday that B2 bomber planes took off from Whiteman Air Force Base in Missouri for the 18-hour mission. They refuelled in mid-air and did not take off from military bases in the Middle East or the Indian Ocean.

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Gen Dan Caine, Chairman of the Joint Chiefs of Staff, said the US attack went largely undetected by Iran's military. He said it was probably the largest B2 bomber strike in US history.

“Bunker buster” is a broad term used to describe bombs that are designed to penetrate deep below the surface before exploding. In this case, it refers to the latest GBU-57 A/B Massive Ordnance Penetrator bomb in the American arsenal. The roughly 30,000 pound (13,600kg) precision-guided bomb is designed to attack deeply buried and hardened bunkers and tunnels, according to the US Air Force.

It is believed to be able to burrow about 60 metres below the surface before exploding, and the bombs can be dropped one after another, effectively drilling deeper and deeper with each successive explosion. It was not clear how many were used in the Sunday morning attack.

The bomb carries a conventional warhead, but the International Atomic Energy Agency has confirmed that Iran is producing highly enriched uranium at Fordow, raising the possibility that nuclear material could be released into the area if it was the GBU-57 A/B that was used to hit the site. However, Israeli strikes at another Iranian nuclear site, Natanz, on a centrifuge centre have caused contamination only at the site itself, not the surrounding area, the IAEA has said. US warplanes also hit Natanz on Sunday.

In theory, the GBU-57 A/B could be dropped by any bomber capable of carrying the weight, but at the moment the US has only configured and programmed its B-2 Spirit stealth bomber to deliver the bomb, according to the air force.

The B-2 is only flown by the US air force, and is built by Northrop Grumman.

According to the manufacturer, the B-2 can carry a payload of 40,000 pounds (18,000kg) but the US Air Force has said it has successfully tested the B-2 loaded with two GBU-57 A/B bunker busters – a total weight of some 60,000 pounds (27,200kg).

The strategic long-range heavy bomber has a range of about 11,000 kilometres without refuelling, and 18,500 kilometres with one refuelling, and can reach any point in the world within hours, according to Northrop Grumman.