Marathon talks aimed at stopping Iran from getting nuclear weapons reach the finish line Tuesday (Mar 31) as global powers scramble to clear the final hurdles hours before a midnight deadline for a framework deal.
After 18 months of tortuous talks, foreign ministers from Iran, the US, China, Russia, Britain, France and Germany hope such an accord will end a standoff that has been threatening to escalate dangerously for 12 years.
An army of technical and sanctions experts worked late into the night in Switzerland on Monday exchanging documents and groping for ways to figure out the outlines of this potentially historic agreement.
"There are marathon meetings happening all over the place. There are several issues that have not been resolved yet. These are important issues," an Iranian negotiator said late Monday.
US Secretary of State John Kerry, in Lausanne since Wednesday in the latest in a series of meetings with Iranian counterpart Mohammad Javad Zarif that have criss-crossed the globe, late Monday said there was still work to do.
"There still remain some difficult issues," Kerry told CNN in his luxury lakeside hotel.
"We are working very hard to work those through. We are working late into the night and obviously into tomorrow."
Kerry said there was "a little more light... today, but there are still some tricky issues. Everyone knows the meaning of tomorrow."
A meeting between Kerry and his counterparts from the other five powers was expected to begin first thing on Tuesday, without Russian Foreign Minister Sergei Lavrov however who left on Monday.
Lavrov will only return if there is a "realistic" chance of a deal, his spokeswoman said earlier.
The threat of new US sanctions, and domestic pressure on Iranian President Hassan Rouhani for his attempts at rapprochement with the West, all but rule out any putting more time on the diplomatic clock.
State Department acting spokeswoman Marie Harf appealed to the Iranians to help overcome the last hurdles. "It's sort of time to see whether they can make these decisions."
Under the deal, due to be finalised by Jun 30, the powers want Iran to scale back its nuclear programme to give the world ample notice of any dash to make the bomb by extending the so-called "breakout" time.
In return, the diplomatically isolated Islamic republic denies wanting atomic weapons and is demanding the lifting of sanctions that have strangled its economy.
Some areas including the future size of Iran's uranium enrichment capacity - a process for making nuclear fuel but also the core of an atomic bomb - appear tentatively sewn up.
But the two sides still appear wide apart on other areas including what to do with Iran's stockpiles of nuclear material and the pace at which sanctions would be eased.
Other tricky issues include the duration of any accord, with Iran resisting demands by the powers to submit to ultra-tight inspections by the UN atomic watchdog for at least a decade.
The powers are also uncomfortable with Iran's desire to continue researching and developing newer centrifuge machines that would enable Iran to process nuclear material more quickly.
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.
"Obviously we always are planning for contingencies," she told reporters, adding: "We will have to take a very hard look at where we are and we will have to decide what happens next."
"No one is thinking about what will happen if there is no deal. No-one has discussed this in the talks. Everyone is focused on finding solutions," the Iranian negotiator said.