Diplomats belonging to major powers met separately with Iran and the US to discuss how to bring both back into compliance with the 2015 nuclear deal that Washington abandoned three years ago.
Iranian President Hassan Rouhani has said that talks in Vienna on rescuing a troubled 2015 nuclear deal had opened a "new chapter".
An Iranian delegation met Tuesday with representatives of the remaining parties to the agreement to discuss how to bring Washington back into it and end crippling US sanctions and Iranian countermeasures.
US President Joe Biden has said he is ready to reverse the decision of his predecessor Donald Trump to withdraw from the agreement and reimpose unilateral sanctions.
But differences remain over the mechanics of the move as Tehran has since responded by suspending compliance with some of its own obligations under the deal.
Neither the United States nor Iran expect fast breakthroughs in the talks that began in Vienna on Tuesday, with European and other diplomats acting as intermediaries.
The United States was not present at Tuesday's discussions because Iran has refused to meet the US delegation so long as its sanctions remain in place.
"A new chapter has just been opened yesterday," Rouhani told a cabinet meeting Wednesday.
"If (Washington) shows it is honest and sincere, that's all we ask... I think we'll be able to negotiate in a short time, if necessary, with the (other parties to the deal)."
In Washington, State Department spokesman Ned Price said that the United States still believed "this is a constructive forum".
"The talks so far have been business-like and they are doing what we envisioned they would," Price told reporters.
"They are affording us a better understanding of Iran's thinking and we hope that in turn Tehran will leave this round of talks with a better understanding of what we might be prepared to do."
He also suggested Washington might be willing to ease some sanctions on Iran beyond those whose removal was mandated by the original nuclear deal, formally called the Joint Comprehensive Plan of Action (JCPOA), but gave no details.
"We are prepared to take the steps necessary to return to compliance with the JCPOA, including by lifting sanctions that are inconsistent with the JCPOA. I am not in a position here to give you chapter and verse on what those might be," he said.
The US delegation has set up at a different hotel in Vienna, with EU negotiators acting as go-betweens.
The deal's remaining parties, Iran, Britain, China, France, Germany and Russia, agreed on Tuesday to form two expert-level groups whose job is to marry lists of sanctions that the United States could lift with nuclear obligations Iran should meet.
At the same time, two groups of experts, on lifting sanctions and nuclear issues, are working "to identify concrete measures to be taken by Washington and Tehran" to restore the deal, Russian envoy Mikhail Ulyanov said.