The row over Iran's nuclear activities centres on its uranium enrichment programme. Tehran says it is aimed at producing low-grade uranium fuel for civilian purposes. But the International Atomic Energy Agency says more inspections are needed to determine whether this is the case.
John Large, an independent nuclear consultant, explains what is at stake.
Why does Iran need a uranium enrichment facility?
There are only two realistic uses for enriched uranium - to produce low enriched uranium fuel for nuclear reactors and, secondly, to produce the highly enriched uranium for the fissile heart of a nuclear weapon.
The Russian-supplied nuclear reactors presently under construction at the southern port of Bushehr are supplied complete with Russian-made fuel, so there is no demand for enriched uranium for these reactors.
Tehran says it is pursuing a nuclear programme for energy production only.
But Iran does not have a significant and established nuclear reactor programme, and in economic and practical terms establishing an enrichment facility cannot be justified.
That means the enrichment facility is almost certainly for a nuclear weapons programme.
Where did Iran get the know-how to build a nuclear industry?
For the enriched uranium offshoot, Iran needs to obtain thousands of tonnes of uranium ore which it has to semi-process into a powdered form called yellowcake.
Then it needs to refine this and convert into a uranium hexafluoride gas which requires a separate chemical plant in itself.
Tehran denies it has a nuclear weapons programme
Finally it has to construct the enrichment works, based on either a membrane or centrifuge plant, which is the size of at least 10 football pitches, and incredibly energy intensive.
Because enriched uranium can be used to manufacture the fissile material for a nuclear weapon of mass destruction, all of these techniques are safeguarded by an international treaty.
So to establish the plants and factories required, Iran has had to undertake a clandestine programme of acquisition of the hardware and technical knowledge. The most obvious candidates for this are Russia and China.
However, Russia has agreed to confine its nuclear interests in Iran to the construction of nuclear power plants and recent agreements between Iran and China have fallen through.
Another possibility is that a deal has been struck with a third nuclear developing state such as Pakistan or North Korea.
It is also possible that some of the equipment and specialised materials needed have been acquired under apparently legitimate contract with the West but which has dual-use capability.
How would we know if weapons were being developed?
There are a number of treaties that require Iran to declare its intent with regard to developing nuclear weapons.
And nations supplying equipment, such as the Russian nuclear power plants presently under construction in Iran, could also be called to task on this.
In the rush to build up a nuclear weapons arsenal, developing nuclear states do not have the time and sometimes wherewithal to put in place the very advanced abatement technology necessary to stop discharges being emitted from the plants.
It is the monitoring of these discharges, both atmospheric and aquatic, that enables other advanced states to monitor and analyse the radionuclide signatures of these discharges and, hence, determine the state of progress in weapons development programme that Iran might have reached.
The energy intensive plants, such as the uranium hexafluoride and enrichment facilities, can also be spotted by satellite thermal imaging.
What would it mean to the region if Iran did have nuclear weapons?
In the immediate geographical region, even the fact that Iran has or is believed to have started a nuclear weapons development programme, may spur other states on to develop nuclear or other weapons of mass destruction to act as a counterbalance or deterrent.
Iran may even have started a nuclear weapons programme because of similar development programmes thought to be under way in neighbouring Iraq.
It is also generally acknowledged that Israel has had a nuclear arsenal for the last decade or more.
What independent scrutiny will there be?
Iran is a signatory to the Nuclear Non-Proliferation treaty, which is overseen by the United Nations watchdog the International Atomic Energy Agency (IAEA).
That means Iran's nuclear facilities have to be declared and can be monitored by IAEA inspectors as well as remote equipment such as cameras.
However, it could follow North Korea's decision in January to withdraw from the treaty.
Key states like India, Pakistan, and Israel have not joined the treaty.