Iranian foreign minister deputies Abbas Araqchi and Majid Takht-e Ravanchi held talks with Under Secretary of State for Political Affairs Wendy Sherman in the first of Saturday's meetings, PRESS TV reports.
The three will later join Iranian Foreign Minister Mohammad Javad Zarif and US Secretary of State John Kerry, who are also due to meet later in the day to find solutions to outstanding issues ahead of a deadline for a comprehensive agreement between Iran and the P5+1 group of countries on Tehran’s nuclear program.
The meeting will also be attended by head of the Atomic Energy Organization of Iran (AEOI) Ali Akbar Salehi, US Energy Secretary Ernest Moniz and EU foreign policy chief deputy Helga Schmid.
Nuclear negotiation moved into top gear Saturday as both sides demanded the other give ground as a deadline loomed.
"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 Friday 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 talks.
He will join US Secretary of State John Kerry, Zarif and negotiators from the six powers, chasing an agreement on the broad outlines of they hope will be a historic deal by Tuesday.
Since a major diplomatic push to resolve the long-running crisis began in 2013, Kerry and Zarif have met multiple times but have twice missed a deadline to nail down an accord.
Iran wants an easing of international sanctions.
On Sunday 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 Saturday, 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 on Saturday morning that "no text has been prepared".
The US State Department official, asking not to be named, agreed the negotiations "have been tough and very serious".
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.
Rouhani also phoned the presidents of Russia, China and France and Britain's prime minister. The US official said Saturday Washington hoped this was "a sign that Iran is ready to make some of the tough decisions" needed.
But global powers insist sanctions will only be suspended, not lifted, and in a phased manner in case Iran violates the deal.
The UN has imposed several rounds of sanctions since 2006 aimed at stopping Iran from expanding its nuclear and missile programmes while EU and US sanctions since 2010 have targeted its oil exports and banks.
Iran's Insa news agency quoted an Iranian negotiator as saying that "solutions" had been reached in the talks over the US and EU sanctions but that the UN ones remained a problem.
"The UN sanctions... have to be lifted completely and immediately," the diplomat said. This is thought to be a no-go for the powers, all except Germany permanent members of the UN Security Council.
Kerry is under pressure to return from Lausanne with something concrete to head off a push by Republicans to introduce yet more sanctions, something which could torpedo the whole negotiating process.