Iran and world powers have made "important progress" towards a final deal aimed at curbing Tehran's nuclear programme in exchange for eased economic sanctions, a senior Iranian negotiator said Thursday.

"We have made important progress on the final text, but less so on the annexes, so work continues," deputy foreign minister Abbas Araghchi was quoted as saying by state media as he arrived in Vienna for the latest round of nuclear talks.

US Under Secretary of State Wendy Sherman will lead America's delegation to the new round of talks Thursday with global powers in the Austrian capital, US officials said.

She was meeting with her counterparts from the P5+1 group on Wednesday, before Thursday's talks.

It had not yet been determined how long Sherman would stay in Vienna, a State Department spokeswoman Marie Harf said.

"We will be sending people out to Vienna, as needed," Harf said. "But I expect this will be where the action is on these talks for between now and June 30."

The new talks open amid uncertainty over whether US Secretary of State John Kerry, who has been deeply engaged in the nuclear issue over the past 18 months, will be able to travel to Vienna after breaking his leg in a cycling accident.

Sunday's fall in the French Alps came a day after Kerry spent six hours locked in bilateral talks with his Iranian counterpart Mohammad Javad Zarif in Geneva.