The big five nuclear powers have issued a joint statement welcoming progress towards worldwide nuclear disarmament, although their optimism remains overshadowed by the prospect of a nuclear-armed Iran.

The US, Russia, China, the UK and France reiterated their “enduring and unequivocal commitment to work towards nuclear disarmament”. The statement came on Friday at the end of two weeks of talks at the United Nations to prepare for next year’s scheduled review of the 39-year-old nuclear non-proliferation treaty (NPT).

Delegates of the 189-member NPT credited the “Obama factor” with having changed the tone of the international debate on nuclear arms. Boniface Chidyausiku, the meeting’s Zimbabwean chairman, spoke of “the goodwill of the new administration in the US and the willingness of the US administration to engage with the international community”.

For the first time in 15 years the meeting succeeded in coming up with an agenda for an NPT review that was acceptable to both the nuclear powers and to states that resent the manner in which they exercise their monopoly.

It failed, however, to agree on recommendations to send forward to the review conference that will also be held in New York. Delegates nevertheless said the talks had broken a long-standing deadlock. “We have 12 months in which to deepen discussion of what really matters – how to elaborate a shared vision of the steps needed to achieve a world free of nuclear weapons,” said John Duncan, UK representative.

During the meeting, delegates who included Rose Gottemoeller, US assistant secretary of state, urged the four nuclear-armed states outside the NPT – India, Pakistan, Israel and North Korea – to join the treaty.

Iran is a member but is accused of using its nuclear power programme to hide efforts to build a bomb. Under the NPT, all member states have the right to adopt nuclear power for peaceful purposes.

Mohamed el-Baradei, the UN’s chief nuclear watchdog as head of the International Atomic Energy Agency, urged Tehran to negotiate with the US on its nuclear programme.

He told the German magazine Der Spiegel at the weekend: “I advise my Iranian negotiating partners: grasp the hand that Obama is extending to you.”

Urging Iran to accept a so-called “freeze for freeze” formula, he said: “The Iranians would install no more [uranium enrichment] centrifuges, the west would forgo further sanction measures. During this time, there would be intensive negotiations.”

The UN Security Council has imposed three sets of sanctions on Iran for failing to halt enrichment. The US administration has said further measures will be implemented if Tehran fails to co-operate.

Diplomats said the more positive mood on disarmament was in part due to better relations between Washington and Moscow. The statement by the five nuclear-armed powers welcomed the decision by the US and Russia to negotiate an agreement to replace the strategic arms reduction treaty, and their moves towards enforcing a nuclear test ban treaty.

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