The foreign ministers of Iran and six world powers are fast approaching the deadline for an agreement aimed at finally ending Tehran's nuclear standoff with the west.
The ministers have been in marathon negotiations in Lausanne, Switzerland to reach a deal.
But according to observers, three major sticking points still have to be resolved.
Among them is the duration of any limits on Iran's nuclear activities after an initial 10 years.
Also, the possible restoration of sanctions against Tehran in the event Iran fails to comply with the deal terms.
Without answers to these questions, diplomats say, there cannot be an agreement.
Even if the parties reach an agreement by tomorrow, officials close to the talks say it could still fall apart.
The two sides still have to agree on all of the technical details for a comprehensive final accord by June 30.