The United States said Thursday that comments by Iranian President Hassan Rowhani had 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."

There is growing speculation about whether Obama or Kerry could meet with Rowhani on the sidelines of the United Nations General Assembly (UNGA) next week. Both presidents are due to address the gathering within hours of each other on Tuesday.

Both men "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."

White House spokesman Jay Carney said there were "encouraging things" had been said by Tehran since Rowhani took over as the president of the Islamic Republic in August.

"There's no question that the new Iranian government has been taking a different approach in the things that it has said about a lot of issues.," Carney said.

"It has taken some actions that suggest a new approach. But actions are more important than words."

Carney also hinted the United States would be carefully watching Rowhani's speech to the general assembly for comparisons to the past annual angry tirades against Israel by former president Mahmoud Ahmadinejad.

Rowhani said in an interview with NBC News broadcast on Thursday that Israel was an "occupier and usurper" that had brought instability to the Middle East.

But he dodged questions about Ahmadinejad's previous denial of the Holocaust, saying he was not a historian.

"We have seen certainly from President Rowhani's predecessor, you know, incredibly offensive statements with regards to Israel and the Jewish people," Carney said.

"We are assessing and evaluating all the things that the new government is saying and doing."

The White House has said there are so far no plans for Obama and Rowhani to meet, but officials have not categorically ruled out such talks.

And Harf added that Kerry's schedule for the week in New York had also not yet been finalized.