Negotiators failed to narrow their differences in talks on Friday, which followed a round of negotiations in February, also in Kazakhstan's commercial hub, Almaty.
The six nations - the United States, Russia, China, France, Britain and Germany - suspect Iran's nuclear program has the covert aim of giving Tehran the capability to make an atom bomb.
The Islamic Republic denies it is seeking nuclear weapons and says it wants nuclear power for electricity generation and medical purposes.
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