Sergei Lavrov is so far the only foreign minister from the P5+1 group — the United States, Russia, China, Britain, France and Germany — to be taking part in the talks.

It remained unclear if his counterparts would also come, as they did two weeks ago in the last round of talks in the Swiss city.

Hopes were meanwhile rising that the negotiations were making progress, with Iranian Foreign Minister Mohammad Jarad Zarif saying there was “room for optimism”.