US Secretary of state Hillary Clinton says an offer by the United State to engage with Iran is not open-ended.
In a foreign policy speech in Washington, Mrs Clinton said Iran must respond to overtures by President Barack Obama now.
She also said the United States admired the energy of Iran's recent presidential election, but is appalled by the way the government quelled protests over the disputed ballot.
Violent street protests broke out in Iran after President Mahmoud Ahmadinejad was re-elected on 12 June.
The BBC reports some 17 people are thought to have died during days of clashes.
Mr Obama has talked of engagement with Iran but has not made clear how that might take place.
Mrs Clinton also warned Iran could face greater isolation over its nuclear programme.
The US fears Iran's nuclear programme is a cover to build atomic weapons, a charge Iranian officials deny, saying it will be used for domestic power.