The United States said Wednesday that the Islamic republic had mounted a "direct challenge" to the UN's atomic watchdog after diplomats in Vienna said Iran had resumed construction and assembly of nuclear centrifuges.

"We are at a very important juncture. In general terms, we need to impress on Iran that trust still needs to be built, and that is up to the Iranians," said one Western diplomat ahead of the talks at the French foreign ministry.

French foreign ministry spokesman Herve Ladsous said the talks would bring together "high-level officials" and were aimed at "establishing trust" with respect to Iran's nuclear program.

Under an agreement reached last year with Britain, France and Germany, Iran agreed to suspend sensitive uranium enrichment, allow tougher inspections and file a comprehensive declaration of its nuclear activities.

The agreement was aimed at allaying international fears that Iran was secretly developing nuclear weapons, a charge that Tehran denies.

But since then, experts from the UN's nuclear watchdog have found omissions in Iran's reporting, inspection visits have been delayed and the regime has backed away from a pledge to suspend all enrichment-related activities.

Diplomats in Vienna said Wednesday that Iran had removed the seals placed on centrifuges by the International Atomic Energy Agency (IAEA) to ensure Tehran was not using its civilian nuclear program as a cover for weapons development.

The centrifuges are used to enrich uranium for use in nuclear power plants, but highly enriched uranium can also be used to make nuclear warheads.

"Actions like resuming making centrifuges do not improve confidence," the Western diplomat said.

In Washington, deputy US State Department spokesman Adam Ereli called reports about Tehran breaking the seals on its centrifuges "disturbing", saying it was a sign that Iran may not be trusted to fulfill its commitments.

"We view it as a direct challenge to the IAEA's call on Iran to suspend all enrichment-related and reprocessing activities," said Ereli.

In Tehran, the deputy head of the Iranian parliament's foreign policy and security commission, Mohamoud Mohammadi, said the assembly would not ratify an additional security protocol to the nuclear Non-Proliferation Treaty (NPT).

Following major pressure from the IAEA and the international community, Iran signed the protocol -- which would give IAEA inspectors increased powers -- in December last year, but has yet to ratify it.

Tehran says it is no longer bound to its deal with the so-called European "big three" because they sponsored a resolution adopted by the IAEA last month, which criticized Tehran for failing to cooperate.

"I hope that Tehran understands that this is not the right way to go," German Foreign Minister Joschka Fischer said Wednesday.

Diplomats said Thursday's talks were part of regular discussions between the Europeans and Tehran, and was not called as a response to recent events.

Ladsous noted that "discussions are ongoing with the Iranian authorities with a view to obtaining guarantees on the peaceful nature of the Iranian nuclear program."

"There are a number of outstanding issues that urgently need to be cleared up. We have to look at ways in which we can do this. So the talks will be mainly dealing with technical issues," said the Western diplomat.

The Western diplomat said the deal between the Europeans and Iran had been successful in that the IAEA "now has a better understanding of the Iranian nuclear program than ever before," with inspectors working on the ground.

But the source warned: "There are outstanding issues which give rise to serious suspicions. Patience is finite and the Iranians have to realize that."

WAR.WIRE