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.

Six global powers -- Britain, China, France, Germany, Russia and the United States -- are trying to nail down a deal to curb Iran's nuclear ambitions by reducing its stockpiles of enriched uranium and mothballing some of its sites before a June 30 deadline.

Negotiations last weekend in Geneva, Switzerland, failed to bridge differences between Washington and Tehran, especially over the crucial issue of inspections of military sites.

Other sticking points remain, including the possible military dimensions of the Iranian nuclear programme and the demands by the world powers for UN inspections of Iranian military bases.

Araghchi said before last week's talks that it would be "out of the question" for UN inspectors to question Iranian scientists and inspect military sites as part of a final deal.

The diplomat said Thursday that reaching a lasting accord would be a "difficult and complicated job".

"The rules are being defined within the framework of the final agreement," he added.