Clinton, visiting Germany for celebrations marking the 20th anniversary of the collapse of the Berlin Wall, said there would be consequences if the Iranian leadership "failed to fulfill their obligations" on the nuclear issue.
The White House later reinforced this position.
"If Iran fails to take steps in its control to demonstrate its responsibility to the world, then sanctions may be necessary," White House spokesman Robert Gibbs told a daily press briefing. Gibbs noted Russian President Dmitry Medvedev had delivered a similar message on Saturday, when he spoke about sanctions during an interview with a German magazine.
Clinton warned Iran would squander a real chance to improve ties with the outside world if it turned away from the plan, which calls for Iran to send low-enriched uranium abroad to be further enriched for use in its medical research reactor.
"We believe that this offer represents an important opportunity for Iran both to meet the medical and humanitarian needs that the Tehran research reactor fills and to begin to restore international confidence in their nuclear program," she told a joint news conference with German Foreign Minister Guido Westerwelle.
Clinton said the countries involved in nuclear talks with Iran — the permanent members of the U.N. Security Council plus Germany — were pursuing diplomacy with Iran as well as a second track that would show the Iranians "there were consequences if they failed to fulfill their obligations."
She said it was premature to discuss what steps might follow or when. Last week in Washington she said negotiators would not wait on Iran "forever."
Her German counterpart Westerwelle echoed her, saying the patience of the international community was "not infinite."
Iran says its nuclear enrichment program is for peaceful purposes, but the United States and other members of the group negotiating with Tehran fear the aim is to produce a nuclear weapon.
(Reporting by David Alexander; additional reporting by Sarah Marsh, and Alister Bull in Washington; editing by Robin Pomeroy and Mohammad Zargham)
