The US secretary of state has said Washington is still willing to engage with Iran but that unrest there means Tehran is currently unable to respond.
Hillary Clinton told the BBC the US was waiting for an answer to its overtures, but Iran did not have "any capacity to make that kind of decision right now".
Mrs Clinton recently warned that Iran's time to respond was limited.
The US accuses Iran of pursuing nuclear weapons but Tehran says its nuclear programme is for peaceful purposes.
"We haven't had any response," Mrs Clinton told the BBC's state department correspondent Kim Ghattas.
"We've certainly reached out and made it clear that's what we'd be willing to do, even now, despite our absolute condemnation of what they've done in the [12 June presidential] election and since, but I don't think they have any capacity to make that kind of decision right now."
The Iranian opposition has accused President Mahmoud Ahmadinejad of rigging the outcome of the poll, which saw Mr Ahmadinejad returned to office.
Days of streets protests against the results were violently suppressed, drawing widespread condemnation from around the world.
One of the most prominent defeated presidential candidates, Mir Hossein Mousavi, has meanwhile announced plans to create a new broad-based political front.
Mr Mousavi said the front would have a charter and would be a way of giving the opposition movement a legal political framework.
Hundreds of people remain in detention in the wake of clashes which erupted after the election results were made public.
Since coming to power at the beginning of this year, US President Barack Obama has spoken of engaging with Iran - in contrast to the policy of his predecessor, George W Bush.
In March, Mr Obama offered "a new beginning" to the Iranian people and leaders, saying his administration was committed to diplomacy.
However, Mrs Clinton said: "The internal debates going on in Iran make it difficult, if not impossible, for them to pursue any diplomatic engagement."
She warned that there was not "an unlimited window of opportunity here - the nuclear clock is ticking and we know we've got to press Iran to begin a serious discussion about its intentions concerning nuclear power".
Tehran insists its programme is solely for civilian purposes, but the US and other Western countries say Iran is covertly pursuing atomic weapons.
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