With the clock ticking on a deadline for finding a resolution to the dispute, the White House said there remained much to do.
"Obviously, the deadline is Monday, and our folks there are working furiously to meet it… Serious gaps do remain," White House spokesman Eric Schultz said.
In Vienna, US Secretary of State John Kerry and Iran's foreign minister extended their stays for a late-night meeting.
Complex deal
Some areas under discussion appear provisionally settled in what would be a highly complex deal that would run for many years, even decades.
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