Senior US and Iranian officials are expected to meet in Oman later for direct talks amid a crisis that has raised fears of a military confrontation between the two countries.
The discussions come after a US military build-up in the Middle East in response to Iran's violent repression of nationwide anti-government protests last month, that human rights groups say killed many thousands of people.
Uncertainty over the location and scope had threatened to derail the talks, which are part of a diplomatic effort by regional mediators to de-escalate tensions.
Both countries remain far apart in their positions. The hope is that, if successful, the discussions could lead to a framework for negotiations.
The US, which is demanding Iran freezes its nuclear programme and discards its stockpile of enriched uranium, had said the talks should also involve Iran's ballistic missiles, the country's support for regional armed groups and its treatment of its citizens.
