The six world powers seeking to negotiate a historic agreement with Iran to curb its nuclear programme plan to carry on negotiating beyond a Tuesday deadline, a senior US official said yesterday.

Iranian Foreign Minister Mohammad Javad Zarif was to leave Vienna and return to Tehran for consultations with the country’s leadership on the state of negotiations, Iran said.

Officials from both sides said big differences still remained. EU foreign policy chief Frederica Mogherini said yesterday it was not impossible to get an accord by the self-imposed deadline but that a few extra days may be needed.

Foreign ministers from the negotiating countries were gathering in Vienna yesterday to assess where the talks stood. They expected them to spill over into July.

There are a number of different areas where we still have major differences of interpretation

“Zarif will return to Tehran tonight and will come back to Vienna tomorrow,” Iran’s Tasnim news agency said, citing an unnamed Iranian official. An Iranian official said Zarif would “consult with the leadership” over the talks in Vienna.

The negotiations aim to limit Tehran’s nuclear programme in exchange for a lifting of US, EU and UN sanctions on Tehran. The US, Israel and some Western nations fear that Iran has been trying to develop a nuclear weapons capability, but Tehran says its programme is for peaceful purposes only. The main differences are on the pace and timing of sanctions relief for Iran and on the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.

US and European negotiators also want to ensure there is a mechanism for restoring sanctions if Tehran fails to meet its commitments under any future accord. In November, the seven nations involved in the talks set a late March deadline for a framework agreement, which they ultimately reached on April 2, and a June 30 deadline for a comprehensive deal.

Highlighting how much work remains, British Foreign Minister Philip Hammond said yesterday major challenges remained, including on the parameters already agreed in April.

“There are a number of different areas where we still have major differences of interpretation in detailing what was agreed in Lausanne,” Hammond told reporters.

“There is going to have to be some give or take if we are to get this done in the next few days,” he said. “No deal is better than a bad deal.”

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