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 because Tehran rejects face-to-face talks for now.

Former US president Donald Trump withdrew from the deal, which lifted economic sanctions on Iran in return for curbs to its nuclear program. He reimposed US sanctions, prompting Iran in turn to violate the accord's atomic limits.

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.

Diplomats said the working groups, which are chaired by the European Union and exclude the United States, met on Wednesday and suggested they would know in a matter of weeks whether they might bear fruit before Iran's June 18 presidential elections.

US officials said they had been briefed on the meetings.

"The team on the ground in Vienna has had consultations with our European allies as well as with our Russian and Chinese partners. They in turn have met with the Iranian delegation," US State Department spokesman Ned Price said in Washington.

The talks would be hard because of the indirect format, the history of US-Iran mistrust and the complexity of the issues, Price said.

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.

Australian Associated Press