Iran's chief nuclear negotiator Abbas Araghchi said Wednesday that "problems" remain in nuclear talks with world powers and there can be no deal without a "framework for the removal of all sanctions".
In a live interview with state television from the talks in Switzerland, Araghchi said that "until we have solutions to all problems we cannot have a comprehensive agreement," naming sanctions and research and development of advanced centrifuge machines as key stumbling blocks.
He said a joint statement on progress made in recent days would be issued later Wednesday in Lausanne, where the negotiations are taking place.
The comments came after Iran and the P5+1 powers -- Britain, China, France, Russia and the United States plus Germany -- missed a March 31 deadline for a political deal meant to pave the way to a final agreement over the Islamic republic's disputed nuclear activities.
Araghchi said the two sides had begun drafting key details on some issues for the basis of an agreement but Iran would not be rushed into concluding the long-running negotiations.
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"We won't let time bind us in the talks," he said. "Time is important to us but the content of the negotiations and our demands are more important."
He said Iran's future plans to use more advanced centrifuges -- the machines that enrich uranium gas by spinning it at supersonic speeds -- was an unresolved issue.
"Sanctions are one of the key issues, and in regard to Iran's nuclear programme the subject of research and development is another key issue that we insist on.
"For sure our research and development of advanced centrifuge machines should continue."
Centrifuges are used to purify uranium for nuclear power generation, but they can also enrich it to higher purities for use in an atomic weapon, which Iran denies pursuing.
With hopes of an agreement hanging in the balance, Araghchi said nothing should be read into the departure from Lausanne of the Chinese, French and Russian foreign ministers.
"They had some other commitments," he said. "The talks will go on as long as necessary."