A top US envoy warned Iran Friday that its pursuit of more advanced uranium-enriching technology would intensify the long-running international standoff over its disputed atomic drive.

"Any Iranian attempt at a more advanced centrifuge would be an escalation of Iran's ongoing non-compliance with its obligation to suspend all enrichment-related activities," the US ambassador to the International Atomic Energy Agency, Gregory Schulte, told AFP.

It would constitute a "further violation of Iran's international commitments, further reason why we are concerned about the nature of Iran's nuclear programme and the intentions of its leaders, and further reason for the Security Council to act," he said.

Media reports have said Iran is testing advanced centrifuges to enrich uranium, in flagrant defiance of UN resolutions to suspend all enrichment activity until the IAEA, can verify that such activities are entirely peaceful.

Enriched uranium is used to make nuclear fuel, but can also be used to make fissile material for atomic bombs.

Schulte said he could not confirm that tests of the new generation of centrifuges were underway at Iran's nuclear facility in Natanz, as media reports had claimed.

Both the IAEA and Iran's ambassador to the agency, Ali Asghar Soltanieh, declined to comment on the matter.

Schulte said would wait for a new report by IAEA chief Mohammed ElBaradei to see exactly what Iran had declared on its advanced centrifuge work.

"While we have no information on the technical nature of any new Iranian centrifuge, we assume the purpose of testing is to increase Iran's potential enrichment capacity," the ambassador said.

Another western diplomat, requesting anonymity, also said that such tests would make it difficult to resolve the Iranian nuclear stand-off.

"The IAEA and the UN Security Council have been absolutely clear the Iran needs to suspend" enrichment, the diplomat said.

"Instead, it is rushing to develop new enrichment technology. This seriously undermines confidence at a time Iran should be doing as much as possible to restore it, given the real lack of confidence that exists."

Iran's refusal to suspend its enrichment activities, in defiance of two sets of UN sanctions and the threat of a possible third, have fuelled western suspicions that Tehran is seeking to develop the atomic bomb.

Iran insists it has inalienable right to develop the technology to generate nuclear power to meet the energy needs of a growing population.

US ambassador Schulte said there was no for Iran to enrich uranium, since Russia is supplying fuel for its Bushehr nuclear reactor.

According to David Albright, head of the Washington-based Institute for Science and International Security, Iran has been developing a new advanced model of uranium-enriching centrifuges in order to overcome technical problems dogging the P1 first-generation, centrifuges used so far at Natanz.

In Washington, Tom Casey, a State Department spokesman, noted that even if this information could not be confirmed, the fact it is being discussed shows Tehran's will to defy UN Security Council demands, and as such justifies adopting new sanctions.

"Certainly the fact that people are out there reporting that they are not only continuing to work with existing centrifuges but seeking to upgrade what kind of equipment that have, just shows that there is absolutely no effort on their part to really move in a direction that the international community wants them to," Casey said.

"And that is why we will have another sanction resolution hopefully in a not too distant future," he added.

Last year, IAEA inspectors confirmed Iran's claim that it had 3,000 P1 centrifuges up and running in Natanz, the amount needed, in ideal conditions, to produce enough material in one year to make a single atom bomb.

The P1 centrifuges are currently estimated to be running at only 10 percent capacity.

Albright said that P2 second-generation centrifuges produce 2.5 times more enriched uranium than P1 centrifuges, meaning only 1,200 of the advanced centrifuges would be needed to produce enough material to make a bomb.

Nevertheless, Iran has had to design and build its own modified version of the P2 because foreign-made parts are difficult to come by given the trade embargo in place against the Islamic republic.

Diplomats have suggested that Iran let IAEA chief ElBaradei see the advanced centrifuges during a visit to Iran last month in a gesture of cooperation.

ElBaradei's report is expected to be released around February 20.