A senior U.S. official at nuclear talks in Vienna says Iran has not moved from what he calls its "unworkable and inadequate" positions.
The official said Saturday there has been some progress but that both sides are still very far apart on some key issues, including uranium enrichment.
Another U.S. official said it would be difficult to extend the talks beyond the July 20 deadline without seeing progress on those issues.
U.S. Secretary of State John Kerry and the British, French, and German foreign ministers will join the talks in Vienna Sunday. The Chinese and Russian ministers are unavailable and are sending top diplomats.
Iranian Deputy Foreign Minster and chief negotiator Abbas Araghchi says it is important to have the foreign ministers at the talks, hoping they can achieve a breakthrough; but, he said the differences are deep.
An interim deal for Iran to curb its nuclear enrichment program in exchange for an easing of Western sanctions expires July 20.
Britain, China, France, Germany, Russia, and the United States are trying to work out a longer term deal with Iran.
The U.S. accuses Iran of seeking to build a nuclear weapon. Iran insists its nuclear program is strictly for peaceful civilian purposes.
Some information for this report comes from AP, AFP and Reuters.