Iran will never slow down its nuclear research programme, its supreme leader said yesterday as negotiators from Tehran and six world powers struggled to narrow “significant gaps” that the US warned might be insurmountable.

The stakes in a deal are high on both sides. Western powers, along with Russia and China, want to prevent chronic tensions in the Middle East from boiling over into a wider war or triggering a regional nuclear arms race. Iran, for its part, is keen to be rid of international sanctions hobbling its oil-based economy.

Clerical supreme leader Ayatollah Ali Khamenei said that the Islamic Republic’s negotiating team in Vienna should not yield to issues “forced upon them”.

“These negotiations should continue,” he told nuclear scientists in Tehran, the official Irna news agency reported. “But all should know that negotiations will not stop or slow down any of Iran’s activities in nuclear research and development.”

Tehran denies suspicions that it has used its declared civilian atomic energy programme as a front for covertly developing the means to make nuclear weapons, maintaining that it seeks only electricity from its enrichment of uranium.

Negotiators from Iran and the so-called P5-plus-one – the US, Russia, China, Britain, France and Germany – plan after their two days of talks in Vienna to start drafting a long-term agreement on settling their decade-old nuclear dispute by a self-imposed deadline of July 20.

They will begin their next round of talks in the Austrian capital on May 13, European Union foreign policy chief Catherine Ashton, coordinating the talks for the powers, told reporters while standing next to Iranian Foreign Minister Mohammad Zarif.

“A lot of intensive work will be required to overcome the differences,” she said after the April 8-9 meeting ended. “We will aim to bridge the gaps in all the key areas.”