TEHERAN (BLOOMBERG) - Iran and world powers have started what could be their final negotiations to revive a 2015 nuclear accord that limited the Islamic Republic's atomic activities in return for sanctions relief.

The fifth round of talks has begun in Vienna, with Iranian and Russian officials saying that while some issues still need to be resolved, it could be the final one.

If a deal is struck, the United States would probably ease sanctions on Iran's oil, banking and shipping sectors, though it is unclear to what extent or how quickly that would happen.

Iran, whose economy has been battered by the US penalties, is preparing to ramp up crude production and boost exports in anticipation of an agreement.

The negotiations have not hit a "dead end", Iranian Foreign Ministry spokesman Saeed Khatibzadeh told reporters in Teheran on Monday (May 31).

"If key issues are resolved, this could be the last round of talks. Otherwise, there will be another round," he said.

Russia's main envoy for the discussions, Mr Mikhail Ulyanov, said the countries involved expected that "the current round should be final".

Iran will hold presidential elections on June 18 and Teheran is keen to conclude the talks before then. Iranian President Hassan Rouhani is stepping down after serving two terms, though he will stay in office until around mid-August.