"What they've agreed is that if Iran does not accept this offer of negotiations or does not negotiate in good faith, we will return to the Security Council," the official said.

The official spoke after the United States, in a major policy shift, offered to participate directly in talks on Iran's disputed nuclear program if Tehran suspended all uranium enrichment activity.

The breakthrough comes as world powers prepared to meet in Vienna to mull the standoff with Iran.

Washington announced its readiness to have its most substantive contact with Tehran since they broke diplomatic ties 26 years ago.