The talks have stalled since mid-March as negotiators seek to return to the landmark accord that curtailed the Islamic republic's nuclear programme in exchange for sanctions relief.

"The negotiations are at a point of deadlock," said the source, adding a deal had been ready back in March but had "slipped away" due to a dispute over the status of Iran's Revolutionary Guards.

"We are quite pessimistic. Iran has made a mistake by playing down the clock," said the source, who asked not to be named.

After Paris earlier confirmed two more French citizens had been arrested in Iran, the source said the lack of progress on the nuclear issue was coinciding with the "intensifying provocations" by Iran.

"Right now there is an interest in returning to the agreement but it will not always be that way," said the source.

EU diplomat Enrique Mora, who chairs the negotiations, is currently in Iran for talks with Iranian officials.