Iran and world powers embark on Tuesday on the Herculean task of transforming an interim nuclear deal into a long-term accord satisfying all sides and silencing talk of war for good.

After a decade of failure and rising tensions, US President Barack Obama has put the chances of such an agreement at not more than "50-50", while Iranian Foreign Minister Mohammad Javad Zarif has predicted "difficult" discussions.

The scheduled three-day meeting in Vienna between senior diplomats from Iran and the United States, China, Russia, Britain, France and Germany - the P5+1 - is the first in what is expected to be a series of tricky encounters in the coming months.

It comes after foreign ministers struck a deal in Geneva on November 24 that saw Iran agree to curb - for six months - some of its nuclear activities in exchange for minor relief from painful sanctions.