The United States' top diplomat warned Iran on Sunday that time is running out on nuclear negotiations between the Islamic republic and world powers.
"This is not an endless process. ... You can't just talk for the sake of talking," Secretary of State John Kerry told a news conference in Istanbul.
Iran and six world powers -- the United States, Russia, China, Britain, France and Germany -- have failed to break the deadlock over Tehran's nuclear drive.
After two days of talks in Almaty, Kazakhstan, aimed at limiting Iran's nuclear programme, chief negotiator Catherine Ashton, the European Union's top diplomat, said on Saturday that the sides were still "far apart."
And no new date was agreed for the resumption of negotiations.
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"This is not an interminable process ... we hope that out of Almaty will come a narrowing of the differences," Kerry said.
The continuing diplomatic deadlock comes as Israel refuses to rule out a pre-emptive strike targeting atomic facilities in the Islamic republic.
Kerry repeated Washington's willingness to have a diplomatic solution.
"It is our desire to have a diplomatic solution but this choice lies in the hands of the Iranians," he said.
Iran insists on international recognition of what it says is its "right" to enrich uranium, a key component of the nuclear fuel cycle which can also be used to make the explosive core of an atomic bomb.
World powers say Tehran must end enrichment to high levels and verifiably suspend operations at the Fordo mountain bunker where such activity takes place before recognising Iran's rights to pursue less threatening activities.