"Out of the 37 tonnes of yellowcake, a few tonnes has been used and converted. This is an experimental and testing process," Hossein Mousavian told AFP.

He was referring to 37 tonnes of uranium yellowcake which Iran had previously said it would be converting into the gas, uranium hexoflouride (UF6), that is fed into centrifuges to make enriched uranium, which can then be used for peaceful purposes or to build a nuclear bomb.

On September 18, the UN nuclear watchdog -- the International Atomic Energy Agency (IAEA) -- passed a resolution calling on Iran to suspend all parts of the nuclear fuel cycle, including conversion activities at its facility near the central city of Isfahan.

But Iranian atomic energy chief Reza Aghazadeh declared in Vienna several days later that it had already begun conversion, and that "tests were successful".

The IAEA is still investigating Iran, amid fears the country could soon have the capacity to develop nuclear weapons.

Mousavian, a deputy to chief nuclear negotiator and top national security official Hassan Rowhani, nevertheless asserted that the conversion activities were going ahead under IAEA supervision.

"The process of testing has from the beginning been under the 100-percent supervision and control of the IAEA ... and every milligramme of this testing process is controlled by the IAEA," he said.

"The testing process is continuing. We have not yet begun industrial-scale production."

He also repeated assertions from other officials that the Islamic republic, which says its nuclear programme is for peaceful purposes, was unwilling to abide by the IAEA call for a suspension of fuel cycle work.

"A suspension in the framework of the resolution is not acceptable, because... enrichment is legitimate and the right of each member" of the IAEA and signatory of the nuclear Non-Proliferation Treaty (NPT), he said.

He added that "the chapter of negotiations is not closed," and that Tehran was "open to further political negotiations on this issue".

"But the key is the recognition of Iran's right to peaceful nuclear technology, including the fuel cycle. Iran will not tolerate discrimination and double standards," he said.