The UN nuclear watchdog, the International Atomic Energy Agency (IAEA) is investigating whether Iran is fulfilling its commitments under the Nuclear non-Proliferation Treaty (NPT). In 2003 North Korea said that it was withdrawing from the Treaty. What is the NPT?
The Nuclear non-Proliferation Treaty is the cornerstone of international efforts to prevent the spread of nuclear weapons and to open up access to the peaceful uses of nuclear technologies as widely as possible.
Neither Pakistan, nor nuclear rival India, are signed up to the treaty
It was ratified in 1970 with the aim of limiting nuclear weapons to the five states that acknowledged having them - the US, Soviet Union (now Russia), China, Britain and France.
The five "nuclear-armed states", all permanent members of the United Nations Security Council, are bound under the treaty not to transfer nuclear weapons or to help non-nuclear states to obtain them.
A total of 187 countries have, so far, ratified the pact.
Non-nuclear signatories agreed not to seek to develop or acquire such weapons.
However, in return, they are given an undertaking that they will helped to develop nuclear energy for peaceful purposes.
This means that they are allowed to acquire the technology to make nuclear power.
The weakness of the treaty is that the technology used to make fuel for a reactor can also be used to make material for a nuclear bomb. A country could either carry out weapons work in secret or simply leave the treaty and make a bomb anyway.
Therefore, in order to prevent the technology from being used for nuclear weapons, the process is monitored by inspectors from the Vienna-based International Atomic Energy Agency, under what is called a Safeguards Agreement.
If there are doubts as to whether a country is carrying out its commitments, the IAEA can insist on tighter inspections, including surprise ones, called an Additional Protocol.
Iran is now subject to an Additional Protocol.
Some key states have refused to join the treaty.
Outside the club
India, Pakistan and Israel are non-signatories and are all believed to have a nuclear weapons capability.
India, long a critic of the perpetual nuclear monopoly the NPT implies, carried out its first nuclear bomb test in 1974.
Delhi and Islamabad launched a series of nuclear weapons tests in 1998, though they have recently agreed to renew a ban on further tests.
And Israel is thought to have developed at least 200 nuclear weapons.
The treaty has also failed to stop the black market nuclear trade centring around Pakistan scientist Abdul Qadeer Khan, which was exposed early in 2004.
However, the NPT regime has had many successes. South Africa and virtually the whole of Latin America have abandoned any nuclear weapons activities.
South Africa secretly built weapons in the 1980s but destroyed them and joined the NPT in 1991.
That year, Iraq was found to be in violation of, following IAEA inspections after the Gulf War.
Up until this time inspectors were only authorised to visit sites that had been declared by NPT signatories. But after the situation in Iraq proved the inspection regime to be inadequate, its powers were strengthened.
IAEA inspectors were then authorised to make special inspections of NPT states, including research and production sites that are undeclared.
North Korea crisis
The biggest crisis facing the NPT came in January 2003 when North Korea announced that it was withdrawing from the treaty. This meant the departure of IAEA inspectors.
North Korea had previously agreed that it would give up any attempt to make nuclear weapons in exchange for nuclear power stations and fuel oil.
Since then there have been talks with North Korea about the future of its nuclear programme with no result.