US Secretary of State Condoleezza Rice has urged the UN Security Council to adopt a resolution imposing sanctions on Iran over its nuclear programme.
Iran produced its first enriched uranium in February
Iran's defiance must be held to account to save the international community's credibility, Ms Rice said.
Her comments come after Tehran said it was taking steps to further develop its nuclear programme.
The UN gave Iran an end of August deadline to stop enriching uranium, threatening sanctions if it did not.
Iran rejects Western allegations that its nuclear programme has a military aspect, and maintains it is enriching uranium only to generate electricity.
China's UN delegate Li Junhua said it was premature to say that the Security Council was in a position to impose sanctions.
But Ms Rice pressed the council to take immediate action.
"For the international community to be credible, it must pass a resolution now that holds Iran accountable for its defiance," she said.
A report carried by the Iranian student news agency, Isna, on Wednesday said it has installed a second centrifuge cascade for uranium enrichment.
The report says Iranian scientists intended to start injecting uranium gas into it within days.
Iran first produced a small quantity of enriched uranium in February. Scientists were running just one cascade then, made up of 168 centrifuges, the machines that spin uranium gas to enrich it.
Now it has emerged that Iran has doubled its capacity by installing a second cascade two weeks ago.
Some reports had suggested Iranian scientists were experiencing technical difficulties. Others said they were going slow, awaiting the outcome of political talks that have now stalled.
Iran had said earlier that it planned to install 3,000 centrifuges at its site in Natanz by the end of this year.
To produce industrial-scale nuclear fuel, tens of thousands of centrifuges would be needed.