World powers have said that Iran is considering their new offer to lift some sanctions if Tehran scales back nuclear activity the West fears could be used to build bombs.
The United States, France, Germany, Britain, China and Russia presented the offer when their first meeting with Iran in eight months began in Almaty on Tuesday and the Islamic state was considering it, the powers' spokesman said.
A source close to the Iranian negotiating team said on Tuesday that Tehran would put forward its own proposal "of the same weight" as that of the other side, but Western officials said it had not done so during the first day of negotiations.
Western officials described the first day of talks as "useful". One said Iran discussed specific aspects of the powers' ideas in bilateral talks with three of them - Russia, Germany and Britain - but gave no indication how Tehran viewed them.
Iranian state television described the atmosphere in the discussions as "very serious".
In their latest attempt to break years of deadlock in the dispute, the powers are offering Iran a relaxation of some of the sanctions that are taking a heavy toll on its economy.
"Hopefully the Iranians will be able to reflect overnight and will come back and view our proposal positively," the spokesman for European Union foreign policy chief Catherine Ashton, who represents the powers in the talks, said.
"The ball is in their court," Michael Mann added after the first day of discussions on Tuesday.
He did not give details of the offer, but other Western officials have confirmed it includes some limited sanctions easing on trade in gold and other precious metals if Tehran closes underground site Fordow where it carries out its most controversial uranium enrichment work.
The facility is used for enriching uranium to 20 percent fissile purity, a short technical step from weapons grade.
Tightening Western sanctions on Iran over the last 14 months are hurting Iran's economy and slashing oil revenue. Its currency has more than halved in value, which in turn has pushed up inflation.
But analysts say the sanctions are not close to having the crippling effect envisaged by Washington and - so far at least - they have not prompted a change in Iran's nuclear course.