Representatives from Iran and six world powers are about to meet in Geneva to finalise the framework for a deal to limit Tehran's nuclear capabilities.
In return, the West is to ease some economic sanctions on Iran.
The euphoria which greeted an agreement in November this year on limiting Iran's nuclear programme has been replaced by the hard slog of highly technical negotiations on how exactly that deal should be put into practice, the BBC reports.
There have been hitches. Iranian diplomats were angered earlier in December when the United States expanded some existing economic sanctions, contrary to Iran's hopes that sanctions would be swiftly eased.
Meanwhile, Tehran suggested last week that it intends to build a new generation of uranium enrichment centrifuges.
The US may view that as a direct violation of Iran's promise to freeze uranium enrichment technology at current levels.