Speaking after Iran and major powers missed a midnight deadline to agree the outlines of a potentially historic accord at talks that stretched into the small hours, Iran's chief negotiator said the Iranians "won't let time bind us in the talks."
"Time is important to us but the content of the negotiations and our demands are more important," Abbas Araghchi said in a live interview with state television from Lausanne.
Iran denies wanting the bomb and its negotiators are under strict orders from supreme leader Ayatollah Ali Khamenei to refuse any curtailing of its programme without relief from painful sanctions.
Araghchi said today a deal was impossible without a "framework for the removal of all sanctions", but global powers want any sanctions relief to be phased and easily reversible if Iran violates the deal.
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The stakes are high, with fears that failure may set the United States and Israel on a road to military action to thwart Iran's nuclear drive.
Russian Foreign Minister Sergei Lavrov and his Iranian counterpart Mohammad Javad Zarif had raised hopes in the early hours of Wednesday morning that the framework deal might be in sight.
Lavrov told Russian media that they had reached "an agreement in principle on all key aspects of the final settlement", while Zarif said he hoped negotiators could "finalise" the framework today.
But Western countries poured cold water on such expectations, with a senior US State Department official saying tersely: "All issues have not been agreed."
Coming away from Lausanne with a deal meant "the Iranians being willing to meet us where there are still issues to deal with," Hammond told British media.