Iran has started to inject uranium feedstock gas into centrifuges at its Natanz nuclear facility, crossing an internationally agreed "red line" on the path to producing the material for atomic weapons.

A senior diplomat from the International Atomic Energy Agency (IAEA) confirmed that researchers at the republic's pilot enrichment plant in central Iran had taken the crucial step, signalling a major escalation in the long-running face-off between Tehran and the West.

Iran has publicly pledged to resume its suspended nuclear programme before the United Nations' watchdog meets in Vienna next month to discuss possible UN Security Council action, which could include sanctions.

The diplomat said Iran had not yet fired up the whole 164-centrifuge cascade but had begun to load some centrifuges with the raw uranium hexafluoride (UF6), which can then be purified into enriched uranium.

He explained that once nuclear scientists had perfected the enrichment technique at the pilot plant, they would be capable of escalating to industrial-scale enrichment with thousands of centrifuges.

Iran says its nuclear program is a peaceful effort to generate electricity and that it only wants to produce low enriched uranium, which is not refined enough for weapons use. The EU and US fear that the programme is a front for weapons development.

Those fears have been heightened by reports that the republic wants to install more than 50,000 centrifuges at Natanz, which could then produce enough enriched uranium for one atom bomb every fortnight.

Inspectors were due to visit the Natanz plant tomorrow for an inspection, following threats that surveillance seals and cameras would be removed.

Iran had suspended its suspect uranium enrichment for three years until talks with an EU-led negotiating team, aimed at winning guarantees that the program is peaceful, broke down last month.

It has, however, been making the feedstock UF6 at a conversion plant in Isfahan since the summer. The West had appeared ready to allow this work, although technically part of nuclear fuel activities, as long as there was no actual enrichment.

Gholam Hossein Elham, a spokesman for the government of President Ahmadinejad, was asked today whether Iran would wait for the IAEA board of governors’ meeting on March 6 to resume industrial-scale enrichment. He replied simply: "No, no."

The IAEA voted last week to report Iran to the Security Council, but left a one-month window for a final attempt to tempt Tehran back to the negotiating table. Proposed talks in Moscow with Russia, which has offered to carry out enrichment on Iran's behalf, today appeared to have been unilaterally cancelled.

Sergei Kislyak, the Russian Deputy Foreign Minister, said Moscow still expected the Iranian delegation on Thursday. Mr Elham said that Iran's government was now insisting that enrichment was carried out within its borders.

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