Six world powers and Iran started a second day of talks in Vienna on Wednesday on Tehran's contested nuclear program, seeking to close a vast gap in expectations about what a final agreement should look like.

The meeting, which began on Tuesday, aims to set out a broad agenda for talks that could in time produce an agreement on the permissible scope of Iran's nuclear activities and lay to rest Western concerns about their possible military dimension.

The negotiations, likely to extend over several months, could help defuse years of hostility between energy-exporting Iran and the West, ease the danger of a new war in the Middle East, transform the regional power balance and open up major business opportunities for Western firms.

Western diplomats said Tuesday's talks were "productive" and "substantive" but had thus far led to no immediate agreements.

"The focus on was the parameters and the process of negotiations, the timetable of what is going to be a medium- to long-term process," one European diplomat said. "We don't expect instant results."

Iranian Deputy Foreign Minister Abbas Araqchi was cited by Iran's English-language Press TV state television on Tuesday saying that dismantling of the country's nuclear facilities would not be part of the negotiating agenda, highlighting a key sticking point in the talks.

As part of a final deal, Iran expects the United States, the European Union and the United Nations to lift painful economic sanctions, but western governments will be wary of giving up their leverage too soon.

Ahead of the talks, a senior US official said getting to a final deal would be a "complicated, difficult and lengthy process."

"When the stakes are this high, and the devil is truly in the details, one has to take the time required to ensure the confidence of the international community in the result," the official said. "That can't be done in a day, a week, or even a month in this situation."

On the eve of the talks both sides played down expectations, with Iran's Supreme Leader Ayatollah Ali Khamenei saying he was not optimistic about the talks.

However, Iranian Foreign Minister Mohammad Javad Zarif, also quoted by Press TV on Tuesday, sounded an optimistic note.

"It is really possible to make an agreement because of a simple overriding fact and that is that we have no other option."

Reuters