The five declared nuclear powers - Britain, China, France, Russia and the United States - have all stopped nuclear testing and support the adoption of the Comprehensive Test Ban Treaty (CTBT).
However, India and Iran say they will not sign the treaty in its present form, partly because it does not commit the declared nuclear powers to a timetable for dismantling their weapons.
Together with Israel and Pakistan, India, which exploded a nuclear device in 1974, is regarded as a "threshold" nuclear power - one which either possesses nuclear weapons or is capable of assembling them at short notice. Both government and opposition parties in New Delhi object to a treaty that would leave China in possession of nuclear weapons while failing to guarantee India's security.
Western countries had hoped that the conference would approve the treaty at a plenary session today and send it to the United Nations in New York for ratification by member-states. However, at a committee meeting yesterday, India's chief representative in Geneva confirmed that her government would block transmission of the treaty to the plenary session. Pakistan said the CTBT talks would fail unless all nuclear "threshold" states signed it. "If one nuclear-capable state stays out of this treaty, the treaty is dead," said Munir Akram, Pakistan's ambassador at the talks.
Western countries, anxious to keep the treaty alive, even in a limited form, may try to send it to the UN General Assembly's next session on 17 September without Indian and Iranian support. Yet diplomats feared the submission of an unapproved text to the assembly might provoke broader opposition from non-aligned countries.
In its present form, the treaty would not take effect unless ratified by the five declared nuclear powers and the three "threshold" states. India says this requirement is unacceptable because it deprives India of the freedom to pursue a sovereign foreign policy.
Iran opposes the treaty on the grounds that it could enable foreign countries to use spy satellites to ensure compliance and demand "on-site" inspections of nuclear facilities. John Holum, the director of the US Arms Control and Disarmament Agency, said last week that Iran was between 10 and 15 years away from acquiring nuclear weapons.