Rice said Tuesday's announcement was further proof that Tehran was not adhering to international regulations.
It was time for the Security Council to take "strong steps to make certain that we maintain the credibility of the international community on this issue", Rice said on Wednesday.
However, the Iranian move did not call for an emergency meeting of the 15-member Security Council, she added.
On Tuesday, Mahmoud Ahmedinejad, the Iranian president, made a triumphant announcement that Iran had successfully enriched uranium for the first time and would now press ahead with industrial scale enrichment.
International condemnation of the announcement by the Islamic Republic has also come from China, Russia, the European Union and Japan.
Rice has asked for 'strong steps'
Wang Guangya, the Chinese ambassador to the UN, said the move to enrich uranium was "not in line with what is required of them by the international community".
However the ambassador also said that talk of military action or sanctions would not be "helpful".
"We are concerned about the event and the way things are developing, so we do appeal once again to all parties to exercise restraint, to act constructively and not to take action that might further aggravate the situation," he added.
Russia had offered to aid Iran's nuclear programme by helping it to enrich uranium on Russian soil for shipment to Iranian power plants - an offer now seems unlikely to be taken up.
The West fears Iran will use
uranium to develop a weapon
Iran has traditionally regarded Russia as a leading nuclear ally and several officials in Tehran have predicted that Russia will veto any punitive action by the UN security council.
Sergei Lavrov, the Russian foreign minister, said the use of force would not solve the problem but did not reiterate Moscow's past opposition to economic sanctions.
George Bush, the US president, has dismissed media reports of an attack being planned against Iran as "wild speculation".
The Security Council last month gave Iran until April 28 to abandon nuclear activities the West fears is aimed at developing a nuclear bomb.