The United States Wednesday said Iran had shown a new level of "seriousness and substance," at talks with world powers on its nuclear program in Geneva.

President Barack Obama has called on Tehran to show it is serious about substantive nuclear discussions following a diplomatic opening pioneered by the new government of President Hassan Rouhani.

Washington's upbeat assessment appeared to be a carefully calculated and encouraging message sent to Iran after the just concluded talks, though Obama spokesman Jay Carney warned Washington did not yet expect immediate progress.

"We found the Iranian presentation very useful. The Iranian proposal was a new proposal with a level of seriousness and substance that we had not seen before," Carney said.

"Having said that, no one should expect a breakthrough overnight," Carney said, adding Washington wanted to examine the technical and complicated proposals offered by Iran in private.

"As the president has said, the history of mistrust is very deep. The onus remains on Iran to come into compliance with its international obligations, and any deal must prove to the international community that Iran's program will be used for exclusively peaceful purposes."

World powers and Iran earlier agreed to hold fresh talks next month on its nuclear program, after Tehran made a proposal that could allow spot checks on its nuclear sites.

Details of the Iranian proposals are sketchy but Tehran has drawn red lines, saying it will not accept demands to suspend uranium enrichment or ship stockpiles of purified material abroad.

But in order to obtain relief from international sanctions which have punished its economy, Tehran said it is prepared to allow "snap" visits by international weapons inspectors to its nuclear sites.

State Department spokeswoman Jen Psaki cautioned however that "there remain differences in what sanctions relief might be appropriate. They had specific and candid discussions about that."

But she refused to go into the specifics of the Iranian proposals, or what Tehran's stand had been on the issue of its suspect uranium enrichment.

"Iran addressed what they saw as the objective of... what they might do as a first step, and also what should be in a final step. But with all that being said, there's, of course, a great deal more work that needs to be done."

Rouhani, who assumed office two months ago succeeding conservative Mahmoud Ahmadinejad, has pledged transparency on Iran's nuclear program in an effort to get UN sanctions lifted.

The United States and other Western powers suspect that Iran's atomic program is aimed at acquiring a nuclear weapons capability, which Tehran denies.

Under Secretary Wendy Sherman who has led the US delegation in the talks, will now brief Congress and the national security team on her return to Washington.

"We are at a different point in this with a new government in place, and we're having a level of conversation that is different from what we had had in the past, " Psaki said.

Asked about lingering distrust of Iran and its motives, Psaki said the US felt the discussions in Geneva have been serious enough to merit further talks.

"We're satisfied enough that we are -- have scheduled a round of technical discussions and another meeting in early November to continue the conversation," she said.