Diplomats from the United States, Britain, France, China, Russia and Germany hope the talks in the Turkish city of Istanbul would cool tensions over the program, which Iran says is for peaceful purposes.

Western powers suspect Iran of seeking to create a nuclear bomb and have tightened sanctions.

Negotiations between Iran and the six countries, known collectively as the Group of Six, ended in a stalemate in January last year.

Iran has said it will propose "new initiatives" in Istanbul.

U.S. President Barack Obama described on Monday the fresh talks as the "last chance" for the dispute to be resolved via diplomacy.

But Iranian President Mahmoud Ahmadinejad said on Thursday Tehran was "standing firm on its fundamental rights and under the harshest pressure will not retreat an iota from its undeniable right."

Israel has hinted in recent months it may carry out a military strike on Iran's nuclear facilities. The United States has also refused to rule out force.

Russian Deputy Foreign Minister Sergei Ryabkov warned against "overblowing" the differences between Iran and the mediators and said the talks had to be "constructive."

"We really need to find a middle course," the al Arabiya television channel quoted him as saying on Friday. "The negotiations are about renewing confidence."

But a source close to the Iranian delegation told the country's state-run Press TV he saw "few encouraging signs" in the remarks of Western officials.