It was scheduled to come up for review on January 4, but the date has been pushed back to later in the year due to the Covid-19 pandemic.

In a rare statement Monday, five of the world's main nuclear powers pledged to prevent the spread of atomic weapons, and to avoid nuclear conflict.

The treaty's key features:

- The treaty came into force in 1970, and 191 countries have signed up to it.

- It has been ratified by more countries than any other arms limitation agreement.

- It was extended indefinitely in 1995.

- Signatories include the five recognised nuclear weapon states -- the United States, Russia, Britain, France, and China -- that are also the five veto-wielding permanent members of the UN Security Council.

- The treaty requires non-nuclear weapon states that have signed it to refrain from developing nuclear weapons. In exchange, the five nuclear weapon states have pledged to "pursue negotiations in good faith" on disarmament and to provide access to peaceful nuclear energy.

- The International Atomic Energy Agency (IAEA) is tasked with ensuring that efforts to develop nuclear energy are not diverted towards making atomic weapons.

- Four non-parties to the treaty are known, or believed, to possess nuclear weapons. India, Pakistan and North Korea have openly tested and declared that they possess nuclear weapons, while Israel is believed to have some 200 atom bombs but neither confirms nor denies this.

- North Korea is the only country to ever withdraw from the treaty.

- Iran is a founding signatory of the treaty, but the status of its nuclear programme is in dispute.

The NPT's three main pillars:

1. Non-proliferation.

2. Disarmament, with all signatories urged to promote nuclear and total disarmament.

3. The right to peacefully use nuclear technology for all non-weapon countries that can demonstrate their nuclear programs are not used for military purposes.

Purpose of review conferences:

Review conferences are held every five years to assess progress on disarmament and the tougher monitoring of nuclear programs worldwide.

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