"I am saying that we are getting good cooperation from Iran," the head of the International Atomic Energy Agency (IAEA) told journalists at the World Economic Forum in Switzerland.

"In the last 15 months we have made good strides in understanding the nature and the scope of its programme," ElBaradei said, indicating the IAEA had no evidence that Iran was developing nuclear weapons through its atomic energy programme.

In Tehran, however, Iran's supreme leader Ayatollah Ali Khamenei warned European powers Britain, France and Germany to take their ongoing nuclear negotiations seriously, threatening otherwise to reconsider its cooperation.

"The Europeans negotiating with Iran should know that they are dealing with a great, cultured nation ... if Iranian officials feel that there is no seriousness in the European negotiations, the process will change," Khamenei was quoted as saying by the Iranian media.

The IAEA has been investigating Iran for two years on US charges that the Islamic Republic is secretly developing nuclear weapons, and US President George W. Bush warned this month he did not rule out military action if diplomacy fails to secure an Iranian agreement not to seek such weapons.

Iran suspended uranium enrichment, the key process that makes fuel for nuclear reactors but also the explosive core of atomic bombs, as a confidence-building measure under a deal clinched in November by the EU three.

Talks between the trio and Tehran on a more comprehensive plan that would include economic ties are continuing, amid reports that the EU had hardened its stance by urging Tehran to completely dismantle its nuclear fuel programme.

But Iran's top nuclear negotiator, Hasan Rowhani, told the country's conservative Mehr news agency on Saturday: "Europeans know that Iran under no circumstances will give up (uranium) enrichment for peaceful purposes."

"The European Union, especially the three countries, know that Iran is firm on its decisions and I do not think the Europeans want the negotiations to reach a dead end," Rowhani said.

ElBaradei meanwhile said he hoped that the dialogue would be successful, saying it was "vital" for the United States to join the talks with Iran.

"This issue will not be resolved without face-to-face negotiations," ElBaradei added, drawing a parallel with US involvement in talks with North Korea on its nuclear ambitions.

ElBaradei also stressed that IAEA inspectors had yet to uncover any evidence of covert weapons development in Iran.

"As long as we have cooperation, and we do not see a smoking gun, the international community should bear with us," ElBaradei insisted.

"We cannot work on the basis of beliefs, we have to work on the facts," the UN nuclear chief said, while emphasising that the IAEA was currently relying largely on its own equipment, inspections, and information gathering.

Its inspectors were receiving no information or evidence from outside sources, he cautioned, adding: "If people have information and on this basis are coming to the conclusion that this is a weapons programme, then I would very much like them to share it."

IAEA inspectors this month visited the previously off-limits Parchin complex, a military site where Washington charges that Tehran is illicitly simulating nuclear weapons testing.

They announced on January 18 that they wanted to return to the key site, although a diplomat described this as standard procedure.

The UN's nuclear chief also warned of deeper systemic problems with the nuclear Non Proliferation Treaty (NPT), the international regime aimed at preventing the spread of nuclear weapons.

The international community's sparring with North Korea and Iran over nuclear issues, the discovery of private nuclear trafficking, and attempts by extremist groups to seek nuclear material all indicated that the system should be overhauled and the IAEA's powers strengthened, ElBaradei said.

Technological developments meant that a determined country could build a nuclear weapon "in a matter of months or a year", he warned, leaving the IAEA little time to react.

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