Mr Obama also said he was pleased at Russia's quick disapproval of Iran's latest move.
But he said it was unclear how China would respond to a new push at the UN Security Council for another round of sanctions against Iran.
China, a UN Security Council member, has called for further talks over the issue.
China and Russia have been reluctant in the past to support international sanctions against Iran.
"How China operates at the Security Council as we pursue sanctions is something we're going to have to see," Mr Obama said.
The five permanent members of the Security Council - the US, Russia, China, France and Britain - have a veto over resolutions, including sanctions.
Iran currently enriches uranium to a level of 3.5% but requires 20% enriched uranium for its research reactor, which is meant to produce medical isotopes. A bomb would require uranium enriched to at least 90%.
The US and its Western allies say Iran is trying to develop a nuclear weapon - a charge Iran denies.
In October, a deal brokered by the International Atomic Energy Agency (IAEA) was thought to have been struck for Iran to send its uranium to Russia and France for enrichment.
But last month, diplomats said Iran had told the IAEA that it did not accept the terms of the deal - though there have since been other, conflicting messages.
Also on Tuesday, US state department spokesman PJ Crowley said the international community was willing to help Iran secure medical isotopes from abroad.
The offer would help to "build some confidence" and show Iran that enriching uranium to 20% purity was "unnecessary", Reuters news agency quoted him saying.
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