Russia's chief negotiator was cautiously upbeat Friday about nuclear talks between Iran and six world powers, saying the number of issues still to be resolved before a June 30 deadline "can be counted on one hand".

"Some complex issues remain but there are fewer of them than a week ago," Russian news agency RIA-Novosti quoted negotiator Sergei Ryabkov as saying at the latest round of talks in Vienna.

The points to be resolved "can be counted on one hand," he said.

Iran and the P5+1 powers -- the United States, Russia, China, Britain, France and Germany -- agreed in April on the main outlines of what would be a historic agreement scaling down Tehran's nuclear programme.

Together with even closer UN inspections, this would make any push by the Islamic republic to develop nuclear weapons -- an aim it denies -- virtually impossible and easily detectable.

In return, painful sanctions on the Islamic republic would be lifted.

The world powers and Iran set themselves the deadline of June 30 to finalise what would be a highly complex accord, and negotiators have been meeting regularly in Vienna and elsewhere in recent weeks.

Last Friday Ryabkov said there had been a "very worrying" slowdown in progress, but Washington insisted that it still aims to nail down the accord by the end of the month.

Foreign ministers from Iran and the six powers, including US Secretary of State John Kerry despite a broken leg sustained in a cycling accident last month, are widely expected to join the talks ahead of the deadline.

Many observers expect the final round of talks to begin in Vienna towards the end of the month. According to Ryabkov, no date has yet been set for a meeting of the foreign ministers.

Iran's chief negotiator Abbas Araghchi, deputy foreign minister, said Tuesday that the deadline was not "sacred" and could be extended by a "few extra days" if needed.

Iran's President Hassan Rouhani on Saturday criticised the six powers for "haggling".