Iran's supreme leader Ayatollah Ali Khamenei on Wednesday instructed his country's negotiators to steer clear of "humiliation" in nuclear talks with world powers that resume next week in Vienna.

"The negotiators must respect the red lines and tolerate neither pressure nor humiliation, or threats," he said in a speech to teachers carried on his website.

"It is unacceptable for the other side to make threats while negotiations are taking place. What use is negotiation in the shadow of threats?" Khamenei asked.

Negotiations seeking a definitive accord on Iran's nuclear programme are to resume on May 12 in the Austrian capital, the European Union and Tehran said Tuesday.

Iran and the P5+1 -- the five permanent members of the UN Security Council plus Germany -- want to turn a framework accord reached in Switzerland on April 2 into a full agreement by June 30.

"What has been done so far does not guarantee an agreement, nor its contents, nor even that the negotiations will continue to the end," Khamenei, who has the final word on all policy matters in Iran, said a week after the preliminary deal.

Khamenei wants punitive international sanctions lifted without Iran having to abandon uranium enrichment for its nuclear programme.