US Secretary of State Hillary Clinton is to warn Iran that the US will not extend its offer of engagement "indefinitely".
In a foreign policy speech to be delivered later, Mrs Clinton will say that Iran needs to respond to President Barack Obama’s overtures now.
If it does not, Iran could face more penalties and isolation over its nuclear programme, she will say.
She will say Iran used "deplorable" means to quash post-election protests.
Violent street protests broke out after President Mahmoud Ahmadinejad was re-elected in the 12 June disputed election.
Some 17 people were thought to have died during days of clashes.
"Neither the president nor I have any illusions that direct dialogue with the Islamic Republic will guarantee success."
Hilary Clinton
Mr Obama has talked of engagement with Iran but has not made clear how that might take place.
Shortly after coming to office in January, Mr Obama said: "If countries like Iran are willing to unclench their fists, they will find an extended hand from us".
In her speech in Washington on Wednesday, Ms Clinton will say: "We remain ready to engage with Iran, but the time for action is now.
"The opportunity will not remain open indefinitely."
The US fears Iran’s nuclear programme is a cover to build atomic weapons, a charge Iranian officials deny.
Enriched uranium can be used to make atomic weapons, but can also be used in nuclear power plants.
Mrs Clinton will say the Bush administration policy of isolating Iran did not stop it moving towards developing nuclear weapons.
"Neither the president nor I have any illusions that direct dialogue with the Islamic Republic will guarantee success.
"But we also understand the importance of trying to engage Iran and offering its leaders a clear choice: whether to join the international community as a responsible member or to continue down a path to further isolation."</p
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