Doubts have been raised about how technically advanced Iran's nuclear programme is, after it emerged Tehran may have used material from China.
Iran's nuclear programme may not be as advanced as it seems
Western diplomatic sources told the BBC the material used in Iran's recent uranium enrichment experiments probably came from materials supplied in 1991.
That was before China joined the Non-Proliferation Treaty, and before it was bound by its export controls.
Iran recently announced it had been able to produce enriched uranium.
This was despite calls from Western powers to suspend the programme because of fears it could lead to the production of a nuclear weapon.
Iran may have used stocks of high-quality uranium gas - or uranium hexafluoride gas - from China to speed up a breakthrough in enrichment, diplomats say.
This allowed them to proclaim Iran's enrichment programme was under way.
Nuclear experts say Iran has had some problems with impurities in its own production of the material.
So it would be logical to use the good quality Chinese material to test out its enrichment machinery, says the BBC's Jonathan Marcus.
The Iranian move had great propaganda value, but it may also have had a clear political purpose: to demonstrate that the Iranian enrichment programme was a reality, our correspondent says.
It may also have put down a marker that in the event of any future deal, Iran's right to conduct at least some enrichment activity would have to be acknowledged, he adds.