As the latest talks on Iran’s nuclear program began Tuesday, the country’s negotiators took a tough opening position, pledging never to dismantle equipment or facilities that the United States and other world powers say raise suspicion that the Islamic republic could produce atomic weapons.

“Dismantling [the] nuclear program 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 United States, are seeking to reach a permanent agreement that would place limits on the nuclear program and lift a decade’s worth of international economic sanctions that have hobbled the Iranian economy.

The deal would replace last fall’s agreement, which capped the program 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, however, the talks represent the most sustained contact for the U.S. 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 U.S. official said Monday, 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 Tuesday, and they 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 U.N. Security Council — the United States, Britain, China, France and Russia — plus Germany.

Michael Mann, spokesman for European Union foreign policy chief Catherine Ashton, the lead negotiator in the talks, said the process will be “intensive and difficult.”

The international consortium wants an agreement that effectively prevents Iran from quickly converting its nuclear program to weapons production or from hiding a parallel program. 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 signaled that it would oppose any such curbs.

“We will seek to reach a comprehensive plan of action,” Araghchi said in an interview with Bloomberg News on Monday evening. “This is difficult but not impossible.”

The temporary deal allows continued enrichment of uranium on Iranian soil but at a level suitable for nuclear energy production, not for weapons. It is expected that a final deal would maintain a similar arrangement.

The placeholder deal provided Iran with as much as $7 billion in sanctions relief and opened the door for international business deals. Although economically divorced from the United States since its 1979 revolution, Iran maintained robust trade with European, Asian and other nations before sanctions were imposed in response to its uranium-enrichment program. Much of that trade has dried up, leading to economic hardship for the Iranian middle class and domestic political pressure to cut a deal.

Iran says its program is entirely peaceful and has no military application. Domestic uranium enrichment has become a matter of national pride, placing Iran in a select international club that the nation says it deserves to join. Critics of the negotiations in Israel and the United States maintain that Iran cannot be trusted with any level of enrichment.

Iran’s supreme leader, Ayatollah Ali Khamenei, set a cautionary tone ahead of the discussions, saying Monday that he had low expectations.

“I am not optimistic about the negotiations. It will not lead anywhere, but I am not opposed either,” he told a crowd in the city of Tabriz, according to the state-run Islamic Republic News Agency.