The US State Department on Thursday played down expectations for talks on how Washington and Tehran might resume compliance with the 2015 Iran nuclear deal.
The department said the US special envoy for Iran, Robert Malley, was likely to return home as the talks broke for the weekend.
State Department spokesman Ned Price said the talks being held in Vienna were "constructive".
"We would also, however, hasten to not allow expectations to outpace where we are," Mr Price said.
Diplomats from major powers have met separately with Iran and the US to discuss how to bring both back into compliance with the 2015 nuclear deal, which Washington abandoned three years ago.
Neither the US nor Iran expect fast breakthroughs in the talks that began on Tuesday, with European and other diplomats acting as intermediaries because Tehran rejected direct talks for now.
The talks are chiefly between experts in two working groups, who are discussing what sanctions the US might remove and what nuclear limits Iran might observe to revive the deal.
The remaining parties to the original nuclear deal – Iran, Britain, China, France, Germany and Russia – are expected to meet on Friday in a group called the Joint Commission that is chaired by the EU.
Mr Price said the expert-level talks "may resume in the coming days, potentially next week".