Iran moved the uranium gas (uranium hexafluoride, UF6) early this month from a conversion facility at Isfahan into the underground Natanz plant in central Iran where it last month started installing the centrifuge machines which enrich uranium, the diplomats told AFP.

Uranium is enriched to be civilian reactor fuel but can also make the explosive core of atom bombs. The UN Security Council imposed sanctions in December to force Iran to halt all enrichment work.

"A container of UF6 was moved to Natanz at the beginning of the month so that they can spin some centrifuges with gas if they want to," one diplomat said in Vienna.

The diplomat said some nine tons of UF6 was in the container, enough feedstock gas to make at least one atom bomb if fully refined.

Another diplomat who closely monitors the Iranian program confirmed the development.

Both asked not to be named due to the sensitivity of the information.

While Iran has not started running the centrifuges, bringing the uranium gas into the plant was a "provocative" act, non-proliferation analyst Mark Fitzpatrick said.

"Running centrifuges with UF6 is not the act of a country that seeks compromise," said Fitzpatrick, of London's International Institute for Strategic Studies (IISS).

The news comes with Iran's nuclear negotiator Ali Larijani set to meet in Vienna on Tuesday with Mohammed ElBaradei, head of the UN nuclear watchdog, amid spiralling tensions over the Iranian nuclear programme.

Iran refuses to abandon enrichment but Larijani has suggested a compromise that Tehran place a formal limit on the degree of its enrichment as a guarantee that it is not seeking an atomic bomb.

ElBaradei is to report this week on whether Iran has honoured a UN Security Council deadline for it to halt the sensitive nuclear fuel work.

The report could lead to tougher measures being imposed on Tehran by the international community.

Meanwhile, in what might be a political signal, Russia announced Monday that Iran's first atomic power plant, which Moscow is building, could be delayed due to financing and equipment supply problems.

Meanwhile, in a sign of a determined effort by Iran to get Natanz working, non-aligned diplomats in Vienna quoted Iranian atomic agency chief Gholamreza Aghazadeh telling them that Tehran is seeking to install one 164-centrifuge unit per week at the underground site.

Aghazadeh told the non-aligned envoys visiting Iran in early February that he hoped "the goal of installation of one cascade (164 centrifuges) per week could be realised," according to a report by the diplomats obtained by AFP on Monday.

Iran started work in late January on installing 3,000 centrifuges at the heavily bunkered Natanz site, as it moves towards industrial-level enrichment of uranium, diplomats have told AFP.

Iran had said a year ago that it had wanted the 3,000 installed by March 2007 but technical problems have apparently delayed this timetable.

"Iran has not really mastered enrichment capabilities yet," said the diplomat who closely monitors the Iranian program.

The diplomat said Iran would have problems picking up the pace from the research level enrichment it is doing above ground at a pilot plant in Natanz to industrial work with thousands of centrifuges at the huge underground site.

"The cascades would run for a day or two but then grind to a halt because too many centrifuges would be breaking down and could not be replaced fast enough," the diplomat said.

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