Iran started a new round of uranium enrichment only days ahead of the United Nations deadline on Thursday for it to stop all such activities, diplomats told AFP.
"They put in small quantities of (feedstock) uranium hexafluoride (UF6) gas last week," into a cascade line of 164 centrifuges in Natanz which enrich uranium, a diplomat close to the watchdog International Atomic Energy Agency (IAEA) said Wednesday.
A second diplomat, who like the first asked not to be named due to the sensitivity of the issue, said the Iranians were doing this "to underscore the point that they are not going to stop enrichment-related activities."
He said the amount of UF6 gas being fed was very small, "under 10 kilos", and that the work was continuing this week.
The UN Security Council has demanded that Iran suspend all uranium enrichment and reprocessing activities by August 31, amid US-led concerns that Tehran’s nuclear programme is a cover for an attempt to produce an atomic bomb.
The IAEA is verifying Iran’s compliance with the deadline and will be sending a report this Thursday to the Security Council.
Uranium enrichment makes fuel for civilian nuclear reactors but in highly refined form can serve as the raw material for atom bombs.
Iran insists its nuclear programme aims solely to produce electricity.
Six world powers have also proposed talks on Iran receiving trade, technology and security benefits if it suspends enrichment.
Tehran said August 21 it was ready for "serious talks", but did mention halting enrichment, and has made clear that it intends to pursue nuclear fuel production.
Russia and China are reluctant to impose sanctions, even though US officials said these two countries have promised to honour a commitment to crack down on Iran if it refuses the conditions for the international benefits package.
US Undersecretary of State Nicholas Burns said Wednesday he expected sanctions to be imposed within a month.
"We believe the sanctions regime will be agreed to in September by the Security Council and we’re going to work towards that with a great deal of energy and termination," he said on CNN.
US State Department spokesman Sean McCormack said senior officials from Britain, China, France, Germany, Russia and the United States would meet in Europe early next week to begin discussing sanctions against Iran if it is deemed to have flouted the UN deadline.
Diplomats said Iran had paused actual enrichment until last week as it had been running the centrifuges without the feedstock gas, which is often done in research work on learning how to enrich.
Iran fed the 164-cascade at Natanz in April and in June, producing small amounts of enriched uranium suitable for fuel, but far below the quality and quantity needed for weapons, the IAEA has reported.
Iran is also running a 10-centrifuge and a 20-centrifuge cascade as part of what is at this point small-scale work, a diplomat close to the IAEA said.
A diplomat said Iran had been feeding the cascades "periodically", even though they could have been using them for enrichment continuously.
"They want to show that they are doing enrichment but they don’t want to upset the applecart by sticking it in the face of the West," the diplomat said.
The IAEA was also expected to report Thursday that Iran is not fully cooperating with its inspections.
Iran earlier this month blocked IAEA inspectors from visiting a key underground site and diplomats said Iranian authorities are making life increasingly difficult for its investigators in other ways, even if the UN watchdog is still able to monitor the country’s nuclear programme.
But one diplomat said IAEA inspectors were able this week to see the underground site at Natanz, where there are no centrifuges yet installed but which is destined to house tens of thousands of the machines for industrial-level enrichment.
The 164-centrifuge pilot cascade is above ground at Natanz.
In addition, Iran is still not providing information the IAEA has sought for more than a year on "outstanding issues", such as its work on improving centrifuges, and possibly military and nuclear-related activities.
The problems are not yet "deemed to be systematic and obstructionist," said a diplomat close to the IAEA.
If they were, the diplomat said, the IAEA board of governors would be required to act on them as violations of the nuclear Non-Proliferation Treaty (NPT).