The US has criticised Iran over its announcement that it can now produce nuclear fuel on an industrial scale.
The Natanz plant is thought to have room for 50,000 centrifuges
"Iran continues to defy the international community and further isolate itself by expanding its nuclear programme," a US official said.
Iranian President Mahmoud Ahmadinejad said on Monday that his country had "joined the nuclear club of nations".
Iran maintains its nuclear programme is purely peaceful but the West fears it wants to build atomic bombs.
A spokesman for the White House's National Security Council, Gordon Johndroe, said: "We are very concerned about Iran's announcement that they entered an 'industrial stage' of nuclear fuel production."
A British official said the Iranian announcement marked "a further breach of International Atomic Energy Agency and UN resolutions".
The European Union also renewed calls for Iran to suspend uranium enrichment, while UN Secretary General Ban Ki-moon urged Iran to follow the Security Council resolution.
The UN has passed two packages of sanctions against Iran for refusing to suspend its uranium enrichment programme.
Iran announced in February that it had set up two cascades of 164 centrifuges each at Natanz. It said it planned to have 3,000 centrifuges by the end of March.
Mr Ahmadinejad did not disclose any figures on Iran's progress
In his speech on Monday, Mr Ahmadinejad did not say how many centrifuges - the machines that spin uranium gas in order to enrich it to levels needed for fuel - were now operational at Natanz.
Ali Larijani, Iran's chief nuclear negotiator with the West, said at Natanz on Monday that Iran had begun injecting gas into many of the centrifuges, again without specifying a number.
Some other officials said 3,000 centrifuges had been installed, the BBC's Frances Harrison at Natanz reports.
The most sensitive areas at Natanz, deep underground, are thought to be halls that can hold up to 50,000 centrifuges.
Mr Larijani also warned that Iran would have no choice but to review its membership of the Nuclear Non-Proliferation Treaty if further pressure was applied by the West over its nuclear programme.
Journalists and diplomats were invited to the event at Natanz, but EU diplomats boycotted them in protest at Iran's refusal to comply with UN demands to end its uranium enrichment programme.
A year ago, Iran announced that it had produced enriched uranium for the first time. It said it had enriched only to 3.5% - the level needed for nuclear fuel but way below that needed to make a nuclear bomb.