The Islamic republic says it urgently needs the fuel for a research reactor in Tehran that makes radioisotopes for medical purposes such as cancer treatment.

But it has so far been reluctant to enter into a UN-brokered deal which would require it to ship its own stockpile of low-enriched uranium (LEU) to Russia for further processing.

Western countries insist Iran do so to prove that its nuclear drive is entirely peaceful as Tehran has long claimed.

However Tehran sees such a deal as a ruse by the West to confiscate its stockpiled uranium and President Mahmoud Ahmadinejad has now ordered the Iranian Atomic Energy Organization to start enriching the uranium to 20 percent on its own.

French Foreign Minister Bernard Kouchner suggested Iran does not have the capacity to enrich uranium to 20 percent for use in its research reactor and he accused the Islamic republic of "blackmail".

The Iranians "do not know how to make fuel" for their existing medical reactor, he told reporters. "For what purposes do they want to enrich it to 20 percent?"

Nevertheless, experts say the process of enrichment is esssentially the same, no matter what level of refinement is required and Iran's plant in Natanz is already churning out LEU.

"The technology for enriching uranium is basically the same whether you are enriching it at 3.5 percent for use in power reactors, or 20 percent for use in the Tehran research reactor, or 93 percent for use in nuclear weapons," said Mark Fitzpatrick of the International Institute for Strategic Studies in London.

British academic Elahe Mohtasham, a specialist in nuclear non-proliferation issues who has worked at the Foreign Policy Centre and the Centre for Defence Studies at King's College, agreed.

"It's not difficult to go from 3.5 percent to 20 percent because it uses the same technology," she said.

The crucial question was whether, after enrichment to 20 percent, Iran would be able to turn the uranium into the fuel rods to put in a reactor.

"Only very few countries, such as France, can convert the uranium into fuel rods. Neither Iran nor the IAEA has ever indicated Iran can make its own rods," Mohtasham said.

But Iran's envoy to the IAEA, Ali Asghar Soltanieh, insisted Tehran had the know-how.

"We do have the capability of going to 20 percent. And we have the capability to work on the fabrication" of the fuel rods, he told AFP.

Iran has officially notified the IAEA of its plans and "invited the agency's inspectors to be present during this whole process".

Asked when Iran's enrichment plant in Natanz would be ready to start enriching to higher levels, the envoy said preparations would begin Tuesday.

But he acknowledged it may be a while before the 20-percent enriched uranium would actually start being produced.

"It takes time but we'll do it," Soltanieh said.

Some observers have expressed doubt whether Iran has mastered the enrichment process sufficiently to be able to produce the fuel it requires.

"The Iranians were seeing technical and operational problems at their plant in Natanz, because the most recent centrifuges have been installed very rapidly," one diplomat said, speaking on condition of anonymity.

"They would be capable of doubling their uranium production overnight. But at the same time, they don't appear to have the technical capacity to turn the 20-percent enriched uranium into the fuel rods for their research reactor."

Another diplomat said Iran would need longer to get the fuel for its research reactor, if it decided to go it alone.

"All this takes time. They'd get the 20-percent enriched uranium quicker through a deal rather than enriching it on their own," one diplomat said.

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