US Secretary of State John Kerry said Monday he and his global counterparts were going to work deep into the night seeking to resolve the remaining "tricky issues" as a deadline loomed within hours.
"There still remain some difficult issues," Kerry told a CNN reporter as he walked in his luxury lakeside hotel in the Swiss town of Lausanne.
"We are working very hard to work those through. We are working late into the night and obviously into tomorrow."
A deadline for the framework of a deal expires at midnight on Tuesday and officials appealed to all parties to make a huge final effort.
"We`ve been negotiating since September of 2013," said acting State Department spokeswoman Marie Harf, referring to the first meeting between Kerry and Iranian Foreign Minister Mohammad Javad Zarif "and it`s sort of time to see whether they can make these decisions."
With all eyes focused on Tuesday`s deadline, Harf said she could not predict what would happen if the outlines of a deal were not agreed in time.
"We have no idea what will happen if we can`t get this done by the 31st. Obviously we always are planning for contingencies," she told reporters on a conference call.
"We will have to take a very hard look at where we are and we will have to decide what happens next, and I don`t want to predict what that outcome will be."
"We just don`t know where we`re going to be at this time tomorrow, and we will really have to see tactically and strategically what makes the most sense going forward. I think we will know a lot more this time tomorrow."
But Harf stressed "there`s still a path to do this" adding there was still a 50-50 chance of a deal.