"We are prepared to talk about what President Obama said in his address at the U.N. General Assembly [Sept. 24], and that is that he respects the rights of the Iranian people to access a peaceful nuclear program," the senior U.S. official said.
"What that is is a matter of discussion," the official told reporters in Geneva, Switzerland, before Tuesday's start of talks between Iran and six world powers.
Iran insists on its right to enrich uranium for civilian purposes and says it wants that right acknowledged as part of the two days of P5-plus-1 talks, involving the five permanent members of the U.N. Security Council -- the United States, Britain, China, France and Russia -- and Germany.
Iranian Deputy Foreign Minister Abbas Araqchi said Sunday Tehran would assure the West it's not making atomic weapons but would not yield to a Western demand it rid itself of enriched nuclear material.
"Shipping out the material is a red line for Iran," he told Iranian state TV.
U.S. allies in the region, particularly Israel and Saudi Arabia, say Tehran should be denied any facilities to either enrich uranium or produce weapons-grade plutonium because of the potential for military uses, The Wall Street Journal said.
The Geneva talks are the first between Iran and the P5-plus-1 since April and the first since the election of Iranian President Hassan Rouhani, who took office in August.
Rouhani has made a priority of easing the crippling Western economic sanctions imposed on Iran over its nuclear activities.
The U.S. official who briefed reporters said the Obama administration was heartened by Tehran's change of tone and believed Rouhani's election signaled a sincere Iranian intention to chart "a more moderate course."
But Washington and its allies want to see concrete steps to limit the pace and scope of Iran's nuclear program, confine its growing stockpile of enriched uranium and be more open about its nuclear activities, the official said.
"We are going to make judgments based on the actions of the Iranian government, not simply its words, although we appreciate the change in its tone," the official said in remarks quoted by The New York Times.
Any sanctions easing would be "proportional to what Iran puts on the table," the official said, adding Iran and the West would probably disagree about what is considered proportional.
Iran is widely expected to offer to scale back its effort to enrich uranium but has stressed it would want quick reciprocal steps to ease sanctions.
Araqchi told the Iranian Students News Agency Sunday Westerners saw trust-building as "taking some steps on the Iranian nuclear issue."
By contrast, he said, "in our view, trust is made when the sanctions are lifted."
The U.S. delegation includes Adam Szubin, a senior Treasury Department expert on economic sanctions.
In Washington, 10 Republican and Democratic U.S. lawmakers wrote to President Barack Obama Friday urging him to increase sanctions against Tehran until it agrees to a complete enrichment freeze.
U.N. resolutions call for Iran to stop enrichment until it addresses international concerns over any military dimension to its program.
"Iran's first confidence-building action should be ... immediate suspension of all enrichment activity," said the letter, released Monday by the office of Senate Foreign Relations Committee Chairman Robert Menendez, D-N.J.