Russia's deputy foreign minister has said that the positions of Iran and world powers in Moscow talks over Tehran's nuclear drive are "hard to reconcile."
The two-day Moscow talks come after two fruitless rounds in Istanbul and Baghdad.
Deputy Foreign Minister Sergei Ryabkov, speaking at the end of the first day of meetings, added that "two days [of talks] will not be enough."
The delegation of the six powers -- the United States, Russia, China, France, Britain, and Germany -- is headed by European Union foreign policy chief Catherine Ashton.
Michael Mann, a spokesman for Ashton, said the EU would not meet Tehran's key demand of scrapping an oil import ban which comes into effect on July 1.
Tehran is represented by chief nuclear negotiator and head of the Iranian Supreme National Security Council Said Jalili.
Jalili also insisted on the formal recognition of its right to enrich uranium for peaceful purposes.
An unnamed EU official told Reuters ahead of the meeting that the international community does not want to see the talks drag on without clear progress.
"We will have to say 'no' to talks for talks' sake," the official said.
The United States and other countries fear Iran's nuclear program is aimed at developing nuclear weapons, a charge Tehran denies.
Iran's navy recently announced it hoped to build a nuclear-powered submarine
, potentially providing it a technical argument for the need to enrich uranium to 20 percent or more, which has been a key point of contention.
Based on reporting by Interfax and Reuters