The head of the UN atomic watchdog has urged Iran to accept a deal over its nuclear programme by the end of the year, and "move beyond sanctions", BBC reported.
Mohammed ElBaradei said the ball was "very much in the Iranian court".
He was speaking as the six major powers negotiating with Tehran were holding talks in Brussels.
They are discussing their response to Iran's rejection of a key part of a deal that would allow it to continue to develop a nuclear reactor.
A compromise suggested by Iran is likely to be rejected, correspondents say.
Friday's meeting involves the UN Security Council's permanent members - Britain, China, France, Russia and the United States - plus Germany.
The West fears Iran is trying to develop a nuclear weapons capacity. Iran insists its nuclear programme is for entirely peaceful purposes.
Mr ElBaradei - who is standing down as head of the International Atomic Energy Agency (IAEA) on 1 December - was speaking in Berlin.
"It is a unique opportunity to move from sanctions and confrontation to the process of building... trust," he said.
"I believe frankly the ball is very much in the Iranian court," he added. "I hope they will not miss this unique but fleeting opportunity."
UN watchdog presses Iran over nuclear deadlock
