Western leaders suspect Iran is working covertly to develop a nuclear weapon. Tehran says its atomic energy program is peaceful. The two days of talks in Istanbul are a follow-up to talks last month in Geneva, the first held in more than a year.

Impatient with what some analysts have called Iran's zigzag diplomacy, the powers are looking for a clear sign from Tehran that it is ready to engage in a way that helps engender trust, even if there is no substantive progress.

Iran's National Security Council issued a statement, quoted by Iranian television, saying the first session of talks on Friday was held in a "positive atmosphere.” Iran's nuclear negotiator Saeed Jalili is the council's secretary-general.

One of Jalili's aides in Istanbul drew a red line round its enrichment activities during the meeting. "We will not allow any talks linked to freezing or suspending Iran's enrichment activities to be discussed at the meeting in Istanbul," Abolfazl Zohrevand said.

"So far this issue has not been discussed, has not been raised or mentioned by the other party," Zohrevand said. "Iran's nuclear rights cannot be discussed."