The talks, which are due to end on June 30, are likely to be extended beyond next Thursday given the complexity of the issues that remain unsolved, senior officials said.

Also on Saturday, U.S. Secretary of State John Kerry said that there is a “lot of hard work to do. We have some very tough issues.”

The main differences are on the pace and timing of sanctions relief for Iran in return for its steps to restrain its nuclear programme and on the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.

U.S. and European negotiators also want to ensure there is a mechanism for restoring U.S., European Union and United Nations sanctions if Tehran fails to meet its commitments under any future accord aimed at ending a 12-year nuclear standoff between Iran and the West.

The United States, Israel and some Western nations fear that Iran has been trying to develop a nuclear weapons capability but Tehran says its program is for peaceful purposes only.