By Sarah Buckley and Paul Rincon
BBC News website
Iran has alarmed the international community by removing the seals at its nuclear fuel research sites - but experts say it is several years away from being capable of producing a nuclear bomb.
There are two routes to producing an atomic weapon: using either highly enriched uranium, or separated plutonium, and Iran could pursue either or both routes.
Regarding uranium, Iran has already embarked on the first step of the purification process necessary to ultimately produce weapons-grade material.
Iran has taken only small steps on the road towards weapons capability
It has produced reconstituted uranium - what is known as "yellow cake" - at its uranium conversion facility at Isfahan.
However, the influential London-based think tank The International Institute for Strategic Studies (IISS) said in a report in September that this was contaminated and was not currently useable.
Supposing Iran solves this problem, it then needs to embark on the process of enriching the uranium.
For uranium to work in a nuclear reactor, it needs only a small amount of enrichment. Weapons-grade uranium must be highly enriched.
Gas centrifuges are one way of enriching uranium.
Iran already has 164 centrifuge machines installed at its pilot centrifuge plant at Natanz, but that is only a fifth of the total it needs before it is fully operational.
The commercial-scale facility could ultimately house as many as 50,000 centrifuges, according to some estimates.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
Mark Fitzpatrick, senior fellow for non-proliferation at the IISS, says Iran has another 1,000 centrifuges dating back to before it temporarily suspended enrichment in 2003. But these have not been tested to ensure they still work.
Tehran might possibly have parts for a further 1,000 centrifuges, Mr Fitzpatrick told the BBC News website.
Frank Barnaby, consultant for the UK security think tank the Oxford Research Group, agrees that Iran does not yet have a critical number of centrifuges in place.
"They don't currently have enough centrifuges working - so far as we know - to produce significant amounts of highly-enriched uranium or even enriched uranium. They would need a lot more," he told the BBC News website.
Even if the plant is made fully operational, it is currently configured to produce low enriched uranium (LEU) rather than the weapons-grade highly-enriched uranium (HEU).
The IISS estimates that, if Iran decided to develop HEU, it could take it between three and five years to make enough for a single nuclear bomb, assuming that it mastered the technology.
But the IISS also says it could take as long as 10 to 15 years, depending on Iran's ability and intentions.
Dr Barnaby agrees.
"The CIA says 10 years to a bomb using highly enriched uranium and that is a reasonable and realistic figure in my opinion," he said.
Iran could alternatively use plutonium to produce nuclear weapons, but this route is also problematic for Tehran, analysts say.
Plutonium can be produced as a by-product of fission carried out by Iran's Russian-built nuclear power reactor at Bushehr.
The IISS says Iran would need to build a reprocessing plant suited to the fuel used in Bushehr and this would be very technically challenging.
But according to Dr Barnaby, useful reprocessing could be carried out over a short period using a suitably equipped chemical laboratory.
Iran is also constructing a heavy-water research reactor at Arak, which Dr Barnaby says would "very efficiently produce plutonium of the sort that is good for nuclear weapons."
But this will not be ready until at least 2014, and probably later, the IISS has said.