Sullivan's comments followed the start this week of a third round of the talks in Vienna in which representatives of Britain, China, France, Germany, Russia and European Union shuttle between US and Iranian delegations.
"I'm not going to characterize the substance of the negotiations at this point because they are in ... an unclear place," Sullivan told an Aspen Security Forum webinar. "We've seen willingness of all sides, including the Iranians, to talk seriously about sanctions relief restrictions and a pathway back into the JCPOA." The acronym refers to the Joint Comprehensive Plan of Action, the nuclear deal's title.
"But it is still uncertain as to whether this will culminate in a deal in Vienna," he said.
The agreement limited Iran's nuclear program to block the development of nuclear weapons. In return, Iran received relief from US and international sanctions.
Former US President Donald Trump withdrew from the accord in 2018, reimposing and adding US sanctions. In response, Tehran began breaching JCPOA restrictions.
President Joe Biden has pledged to return to the deal. Iran refused direct talks on resuming compliance in exchange for the lifting of US sanctions.
Sullivan was asked whether the Iranians are negotiating in good faith.
"I guess good faith is always in the eye of the beholder," he said. "The Iranians have come in a serious way to have serious discussions about details and the teams are working through those details now."The main differences are over what US sanctions will need to end, the steps Iran must take to resume its obligations to restrict its nuclear program and how to sequence the process.