US Secretary of State John Kerry on Tuesday urged Iran to use "all possible effort" to prove that its nuclear programme was peaceful as the final round of talks was to begin in Vienna.
"This is a very critical week obviously in Iran negotiations," Kerry told reporters in London ahead of talks with British Foreign Secretary Philip Hammond.
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"We hope we get there but we can't make any predictions," he added. "It is imperative that Iran works with us with all possible effort to prove to the world that the programme is peaceful".
Iran and six world powers begin the final round of talks on Tuesday, six days before a deadline for a deal with differences still considerable despite months of negotiations.
If the sides do strike an accord, it would consign to history a 12-year standoff between the West and the Islamic republic over its nuclear programme by making any Iranian drive to make atomic weapons virtually impossible.