The US says comments by Iranian President Hassan Rowhani have opened up a possible new path for diplomacy, and urged Tehran to engage "substantively" with the West.
In an interview, Rowhani suggested a new flexibility in Tehran's approach to nuclear talks which have dragged for years as the international community seeks to rein in Iran's suspect nuclear program.
"I think Rowhani's comments have been very positive, but everything needs to be put to the test and we'll see where we go," US Secretary of State John Kerry told reporters.
"At the right moment, I think the White House and the State Department will make clear where we're heading."
The White House also noted "a lot of very interesting things" coming out of Iran, but President Barack Obama's spokesman warned "actions are more important than words".
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There is growing speculation about whether Obama or Kerry could meet Rowhani on the sidelines of the UN General Assembly next week.
Both presidents are due to address the gathering within hours of each other on Tuesday.
French President Francois Hollande said meanwhile that Rowhani had requested a meeting in New York to discuss the conflict in Syria. It would be the first such meeting between the two nations's heads of state in many years.
Obama and Kerry both "believe that there is an opportunity for diplomacy right now," State Department spokeswoman Marie Harf said.
"We hope the Iranian government takes advantage of this opportunity. The world has heard a lot from President Rowhani's administration about his desire to improve Iran's relations with the international community, and President Obama and his secretary certainly believe that we should test that assertion."
