State Department spokesman Sean McCormack said such steps would probably have to be in the form of a new UN Security Council resolution.
McCormack discussed the U.S. strategy as senior diplomats from the permanent five Security Council members, plus Germany, are meeting in London to discuss tightening UN sanctions on Iran.
A statement from Britain's Foreign Office said the diplomats held "productive" and "serious" discussions" on the next steps to take. The statement said world powers remain committed to seeking a negotiated solution to the crisis.
The British statement said no decisions were made today and that talks will continue later this week. The venue was not disclosed.
On February 25, Iranian President Mahmud Ahmadinejad said Tehran has the technology to produce nuclear fuel. He compared Iran's nuclear program to a train that "has no brake and no reverse gear."
The UN's nuclear agency, the International Atomic Energy Agency, reported last week that Iran had ignored a Security Council ultimatum to freeze its enrichment program and instead had expanded the program by setting up hundreds of centrifuges.
In December, the Security Council imposed some sanctions on Iran over its refusal to suspend enrichment and gave it 60 days to halt enrichment. That grace period expired on February 21.
THE COMPLETE PICTURE: RFE/RL's complete coverage of controversy surrounding Iran's nuclear program.
An annotated timeline
of Iran's nuclear program.