The White House has rebuffed Iranian President Mahmoud Ahmadinejad's offer of a public "face-to-face" policy debate with US President Barack Obama.
The White House said the offer showed new UN sanctions were having an effect.
"We have always said that we would be willing to sit down and discuss Iran's illicit nuclear programme if Iran is serious about doing that," spokesman Robert Gibbs said.
"To date, that seriousness has not been there."
In a televised speech to expatriate Iranians, Mr Ahmadinejad said he planned to be in the US for the UN General Assembly next month.
"We are ready to sit down with Mr Obama face-to-face and put the global issues on the table, man-to-man, freely, and in front of the media and see whose solutions are better," Mr Ahmadinejad said.
"We think this is a better approach"
The two countries have not had direct diplomatic ties for more than 30 years.
Mr Ahmadinejad's move comes after fresh sanctions by the international community over Iran's nuclear programme.
The US and its allies accuse Tehran of seeking to develop nuclear weapons. Iran insists its nuclear programme is solely for peaceful purposes.
"Those sanctions are beginning to have an impact, or else the Iranian government would not be changing its position so often about discussing its programme," Mr Gibbs said.
The Iranian leader, who maintains that US policy favours its rival, Israel, said that former US President George W Bush had never taken Iran's offer up because he was "scared".
"We are ready to have high level talk based on mutual respect and dignity... but if you think you can brandish a stick so that we accept all that you say, that will not happen," he said, in front of a cheering crowd.
He said that Mr Obama "overly values Zionists" and had missed taking advantage of "historic opportunities" to change relations between the two countries.
Shortly after coming to power, Mr Obama had extended an offer of diplomacy towards Iran, but relations have since deteriorated.