Making the powder, uranium tetrafluoride (UF4), is as far as Iran can go in enrichment, according to an agreement reached last month with Britain, France and Germany and endorsed by the UN watchdog International Atomic Energy Agency (IAEA).

UF4 is the precursor to UF6 gas that is fed into high-spinning centrifuges in order to filter out enriched uranium.

Enriched uranium can be fuel for civilian nuclear reactors but also, in its highly refined form, the explosive core of atomic bombs.

Iran was already processing 37 tons of the uranium ore known as yellowcake into UF4 when it struck the agreement with the European trio.

Iran was allowed to finish doing this at its uranium conversion plant in Isfahan, since it is otherwise difficult to clear the conversion machine, which is a mass of tubes and compartments, a diplomat told AFP.

The diplomat said there have apparently been "some technical problems with the plant" which have slowed down Iran's effort to finish processing the yellowcake.

"It may take weeks to finish this," a Western diplomat close to the IAEA said.

The IAEA, which is monitoring Iran's suspension of uranium enrichment, had reported last month that Tehran indicated it would bring material at Isfahan into a "safe, secure and stable state not beyond UF4."