Israel on June 13 launched an unprecedented attack against its arch-foe it said was aimed at preventing Iran from acquiring nuclear weapons -- an ambition Tehran denies.

But building and delivering a nuclear weapon requires several complex steps.

- Uranium and the enrichment process -

The key ingredient in a nuclear bomb is enriched uranium -- or plutonium, which can be obtained through the combustion of uranium.

Although uranium is a relatively common mineral, more than 85 percent of uranium production comes from six countries: Kazakhstan, Canada, Australia, Namibia, Niger and Russia, according to the World Nuclear Association.

Natural uranium is composed of uranium 238, which makes up 99.3 percent, and uranium 235, the remaining 0.7 percent. Only uranium 235, called "fissile uranium", can be used for nuclear fuel.

Enrichment refers to the process of increasing the proportion of U-235 in order to obtain enough fuel to make a nuclear bomb.

But first, uranium ore is crushed and ground before being irrigated with sulphuric acid. Then, groundwater and oxygen are injected into the rock to extract the uranium.

After drying, the result is a concentrated solid known as "yellowcake", which is transformed into uranium hexafluoride, and then heated into a gaseous state to prepare it for enrichment.

The most common process for separating the heavier U-238 from the lighter U-235 involves the use of a series of centrifuges that spin the uranium at high speeds.

Thousands of centrifuges are needed to obtain a sufficient volume of enriched uranium. Only a handful of countries have such installations, which are vast and costly.

According to the Institute for Science and International Security (ISIS), a US-based organisation specialising in nuclear proliferation, Iran has about 22,000 centrifuges, significantly up from the around 6,100 permitted under the 2015 nuclear deal it struck with world powers.

With low concentrations of U-235 -- 3.5 to five percent -- the fuel can be used to power a nuclear energy plant.

Uranium enriched up to 20 percent can be used to produce isotopes for medical uses, for example in diagnosing certain cancers.

To build a bomb, enrichment must be pushed to 90 percent. Such a high concentration -- termed weapons-grade -- is needed for the critical mass to set off the chain reaction leading to a nuclear explosion.

- Nuclear fission -

According to the International Atomic Energy Agency (IAEA), 42 kilogrammes of highly enriched uranium is needed for one nuclear weapon.

Iran theoretically has enough near-weapons-grade material, if further refined, for more than nine bombs.

Atomic bombs work on the principle of nuclear fission where energy is released by splitting atoms, causing a highly explosive chain reaction.

Delivering a bomb aboard a missile, poses further challenges: it entails mastering both ballistics -- all the calculations involved in getting the warhead to its target -- and the miniaturisation of the nuclear charge so that it can be mounted on the warhead.

Only two nuclear bombs have been used militarily: the one dropped on Hiroshima on August 6, 1945 and the one dropped three days later on Nagasaki, which in total killed some 214,000 people.