Two United Nations nuclear inspectors began a week-long trip to Iran on Tuesday to visit the country's Natanz uranium enrichment facility, a day after Tehran declared it had begun industrial atomic work at the site.

An Iranian official confirmed the inspectors' arrival to Reuters and said they were on a routine visit. He gave no further details.

Inspectors from the UN watchdog, the International Atomic Energy Agency (IAEA), routinely visit Natanz in central Iran and other declared nuclear sites although Tehran curtailed more intrusive snap inspections last year in retaliation for its case being sent to the UN Security Council.

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Western nations fear Iran may divert fuel towards a covert military program aimed at building atomic bombs. Iran denies this, saying the fuel would only be used in atomic reactors to generate electricity.

Meanwhile Tuesday, Russia's Foreign Ministry said it has no evidence that Iran has made any technological breakthroughs to allow it to enrich uranium on an industrial scale.

Asked about Tehran's announcement that it had begun industrial enrichment, the ministry said in a statement: "We are not aware of any technological breakthroughs in the Iranian nuclear program recently which would change the nature of work on enrichment being carried out in the country.

Iranian President Mahmoud Ahmadinejad announced during a visit to Natanz on Monday that Iran had begun industrial-scale nuclear fuel production, a fresh snub to the UN Security Council, which has slapped sanctions on Tehran for refusing to halt such work.

Iranian officials said Iran had started injecting gas into a batch of 3,000 atomic centrifuges that are being installed at Natanz. But they gave no figures for the number of machines set up and running, saying U.N. inspectors would confirm numbers.

The semi-official Fars news agency said the IAEA inspectors would stay in Iran for one week.

Uranium hexafluoride (UF6) gas is injected into centrifuges which spin at high speeds and enrich uranium for power plant fuel or, if enriched to much higher levels, for bomb material.

Iran insists its work is peaceful, has refused to stop the work and says it is ready for talks to reassure the West about its intentions. But it says it will not accept preconditions for those negotiations.

The United States and others have insisted they will not negotiate until Tehran suspends enrichment.

"We have passed the stage of setting conditions for talks ... We believe that other parties should move forward based on new realities," Foreign Minister Manouchehr Mottaki told a news conference with a visiting Afghan official on Tuesday.

"We have always said we are ready for talks if they have something new to say. We are fully prepared for talks without preconditions to reach a solution," he added.