Iranian negotiators took a tough opening position as the latest talks on its nuclear programme began.
They pledged to never to dismantle equipment or facilities that the US and other world powers say raise suspicion that the Islamic republic could produce atomic weapons.
"Dismantling the nuclear programme is not on the agenda," Iranian Deputy Foreign Minister Abbas Araghchi said as the high-stakes talks began in Vienna.
Iran and six world powers, including the US, are seeking to reach a permanent agreement that would place limits on the nuclear programme and lift a decade's worth of international economic sanctions that have hobbled the Iranian economy.
The deal would replace last year's agreement, which capped the programme temporarily and rolled back some of its most worrisome elements. The final deal is supposed to be completed in six months, although that deadline could be extended. All sides have said it will be extremely difficult to produce.
Already the talks represent the most sustained contact for the US and Iranian governments in more than 30 years, and they offer a glimpse of a possible rapprochement.
"We have decades of mistrust between our countries, and you don't overcome that even with a very good first step of a nuclear agreement," a senior US official said on the eve of the talks.
"Do we understand each other perhaps a little bit better? Yes. Do we have ways to communicate with each other we've never had before? Yes. But we still have a very long way to go."
Negotiators for the United States and Iran met privately on Tuesday and also attended a large group meeting to formally open the bargaining period. Talks are expected to continue on and off through the spring. Participants include the five nuclear powers that hold permanent seats on the UN Security Council - the United States, Britain, China, France and Russia - plus Germany.
The international consortium wants an agreement that effectively prevents Iran from quickly converting its nuclear programme to weapons production or from hiding a parallel programme. In practice, that is likely to mean a demand that advanced centrifuges for enriching uranium be destroyed or mothballed, and that Iran make changes to a nuclear facility under construction so it cannot produce plutonium.
Iran signalled that it would oppose any such curbs.
Iran's supreme leader, Ayatollah Ali Khamenei, set a cautionary tone ahead of the discussions, saying on Monday that he had low expectations.