"There are four-and-a-half weeks left to conclude a definitive pact. We are hoping for that but the chances are very slim," the diplomat said.
"The ball is in Iran's court" and Tehran would have to make "significant gestures," the diplomat added.
The six powers in the talks with Iran -- Britain, China, France, Russia, the United States plus Germany, known as the P5+1 -- have set November 24 as the deadline for a comprehensive deal.
The aim is to close avenues towards Tehran ever developing an atomic bomb, by cutting back its enrichment programme, shutting down suspect facilities and imposing tough international inspections.
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In return, the global community would suspend and then gradually lift crippling economic sanctions imposed on the Islamic republic.
But the two sides, despite long-running talks, remain far apart on how to reconcile their objectives.
The diplomat said the divisions between two sides remained great.
US Under Secretary of State Wendy Sherman has spoken of "a forest of distrust" between the West and Iran.
But in months of discussions since an interim agreement struck last November took effect in January, some progress has been made.
This includes possible changes in the design of an unfinished reactor at Arak so that it produces less weapons-grade plutonium, enhanced UN inspections, and alterations to Iran's fortified Fordo facility.