"We're at that point in the negotiations where we really need to see decisions being made," a senior US State Department official said late yesterday at the talks in Lausanne, Switzerland.
"The work is very complicated and difficult. The other side needs to choose between pressure and a political accord," countered Iranian Foreign Minister Mohammad Javad Zarif.
France's top diplomat Laurent Fabius, the most hawkish in the P5+1 group of countries negotiating with Iran since late 2013, will be the first European minister to fly in for the crunch talks.
Since a major diplomatic push to resolve the long-running crisis began in 2013, Kerry and the US-educated Zarif have met multiple times but have twice missed a deadline to nail down an accord.
Also Read
The powers want Iran to shrink its nuclear programme in order to make any dash to make a bomb under the guise of its civilian atomic programme all but impossible and easily detectable.
Tomorrow China's Foreign Minister Wang Yi and Russian counterpart Sergei Lavrov will reportedly fly in, as well as the EU's foreign policy chief Federica Mogherini.
Kerry, Fabius and German Foreign Minister Frank Walter Steinmeier will have a working lunch today, a US official said. Britain's Philip Hammond said was on stand-by to come.
The emerging accord is to be rounded out with complex technical annexes by a June 30 deadline.
But Iran's Deputy Foreign Minister Abbas Araghchi told reporters today morning that "no text has been prepared".
President Barack Obama and German Chancellor Angela Merkel also upped the pressure on Iran.
"Iran must make the decisions necessary to resolve several remaining issues," a statement said after the two leaders spoke by telephone.
Iranian President Hassan Rouhani called Thursday for the "unjust" sanctions choking the country's economy to be lifted as he wrote letters to the leaders of all six countries.