Fischer said any resumption of enrichment activities "would lead to a collapse of the talks" designed to win guarantees that Iran is not making nuclear weapons and lead also to a push by the EU and the United States to take Iran before the UN Security Council for possible international sanctions.

UN nuclear chief Mohamed ElBaradei meanwhile made a plea for Iran and the European Union to keep their nuclear talks alive and urged Tehran to refrain from uranium enrichment activities.

Fischer said: "It is the foundation of the talks that uranium enrichment remains suspended." Enrichment makes fuel for nuclear power reactors but also what can be the explosive core of atom bombs.

The Iranian crisis has developed into a serious threat to the nuclear Non-Proliferation Treaty (NPT), which since 1970 has mandated the world fight against the spread of nuclear weapons, and it dominated discussions Monday.

Both ElBaradei and UN Secretary General Kofi Annan warned in speeches opening the month-long NPT conference that the treaty is out of date in the face of new threats and technology and needs to be fixed.

But the Iran question emerged as a crucial topic, with the EU initiative now at a particularly delicate point.

"I hope that both parties will continue to talk," ElBaradei told reporters about the EU effort.

"I would hope that the Iranians will not take any unilateral decisions to initiate any activities that now are currently suspended. I think that any future move has to be agreed between both parties," he said.

Iran agreed in November to freeze uranium conversion and enrichment in order to start talks with EU negotiators Britain, France and Germany.

Fischer said "the Iranians know themselves what the alternative would be in this case (if they resumed enrichment activities), namely a call to the Security Council."

The United States charges that Iran is using an allegedly peaceful civilian nuclear program as a cover for making atom bombs and has called for Iran to be taken to the Security Council for possible international sanctions if the EU diplomatic initiative falters.

Fischer made the European objective clear when he said the goal of the talks was that "there will be a permanent cessation of the enrichment process."

Iranian Foreign Minister Kamal Kharazi, who is attending the conference, refused to say what Iran would do but said he had an "excellent" meeting with Fischer.

"We were discussing ways and means of how to move ahead. Still we have to continue our negotiations," Kharazi said.

He said no further meetings with the Europeans had been scheduled after the European Union failed to agree in London on Friday to Iran's proposal to resume relatively low-scale enrichment.

Iran is unhappy with the progress of the talks, which promise trade, political and other benefits, and may resume uranium conversion activities, which makes the gas feedstock for enrichment, in defiance of the suspension, top negotiator Hassan Rowhani said over the weekend.

In Washington, Secretary of State Condoleezza Rice met French Foreign Minister Michel Barnier, and said she had reiterated US "support for the European trio's negotiations with Iran."

Nuclear crises in Iran and North Korea, the discovery of an international smuggling ring supplying their programs, and the threat of atomic terrorism are among recent developments leading to doubts about whether the NPT is working.

"The plain fact is that the (non-proliferation) regime has not kept pace with the march of technology and globalization, and developments of many kinds in recent years have placed it under great stress," UN chief Annan said.

North Korea kicked out inspectors from ElBaradei's International Atomic Energy Agency (IAEA) in December 2002, withdrew from the NPT the following month, and now claims to have made atom bombs.

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