A senior U.S. official has joined nuclear talks between Iran and six world powers with little more than a month until the deadline for a deal.

The talks resumed in Vienna Wednesday, with negotiators working on a main document and technical annexes outlining how it will be implemented.

Most recent meetings have involved technical experts rather than diplomats. But U.S. Undersecretary of State Wendy Sherman has joined two Iranian deputy foreign ministers and a senior European Union official for this round as the talks move into the final stretch.

The United States, Russia, China, Britain, France and Germany seek long-term curbs on Iranian nuclear programs that could be used to make weapons. Tehran denies such aims but is negotiating in hopes of sanctions relief.