Speaking as he arrived in Vienna for the negotiations, Mohammad Javad Zarif warned that an accord would only happen if the other side -- the five permanent members of the UN Security Council plus Germany -- make no "excessive demands".
"If, because of excessive demands ... We don't get a result, then the world will understand that the Islamic Republic sought a solution, a compromise and a constructive agreement and that it will not renounce its rights and the greatness of the nation," Zarif told Iranian media.
In a joint news conference, British Foreign Secretary Philip Hammond called for more "flexibility by the Iranians to convince us that their intentions in their nuclear programme are entirely peaceful."
The mammoth accord being sought by Monday's deadline, after months of negotiations, is aimed at easing fears that Tehran might develop nuclear weapons under the guise of civilian activities.
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It could resolve a 12-year standoff over Iran's atomic programme, silence talk of war and help normalise Iran's fraught relations with the West after 35 years of mistrust and antagonism.
US and Iranian negotiations are under domestic pressure not to give too much away, however, while Israel -- the Middle East's sole if undeclared nuclear-armed power -- and others in the volatile region are sceptical.