Mahmoud Ahmadinejad, the Iranian president, has said that the country has installed 3,000 uranium-enriching centrifuges at a nuclear plant.
Tehran had said it aimed to have that many centrifuges by the end of July, which are set up in interlinked networks, or "cascades", of 164 machines.
However, diplomats in Vienna say Iran appears to have fewer than the target of 3,000 centrifuges operating.
They also say that Iran's nuclear work appears to have slowed down, possibly due to technical glitches or for fear of provoking world powers into passing a third UN sanctions resolution.
Iran has denied any slowdown.
Disputed figures
On Sunday, Ahmadinejad said: "They [world powers] thought that by issuing any resolution Iran would back down. But after each resolution the Iranian nation took another step along the path of nuclear development.
"Now it has put into operation more than 3,000 centrifuges and every week we install a new series."
A UN atomic energy agency report released last week has said that Iran was still short of 3,000 centrifuges.
The report said that, as of August 19, Iran had a total of 1,968 centrifuges operating at its uranium-enrichment plant in the central town of Natanz.
A total of 656 more centrifuges were said to be under development.
Concerns
Western powers have accused Iran of seeking to build atomic weapons.
They are particularly concerned about progress in the country's uranium-enrichment programme because the process can be used to make fuel for power plants or, if desired, nuclear weapons.
Two sets of sanctions have already been given to Iran for its failure to obey a UN demand to stop enrichment activity.
The US, leading efforts to isolate Iran, has said that Tehran faces a third if it does not stop the work.