US Secretary of State John Kerry says "very real gaps" persist between world powers and Iran over the contours of a nuclear agreement as a Sunday deadline for a deal approaches.
Mr Kerry is citing some progress after two days of nuclear talks with Iran and European partners of the US in the Austrian capital of Vienna. But he says much work remains.
The US politician says he is returning to Washington today to consult with President Barack Obama and Congress about the state of negotiations.
He says he will also discuss the possibility of extending the talks past the deadline so negotiations can continue.