Kerry and Zarif, who have met in Swiss city to discuss technical details, aim to reach a definitive agreement by July 1.
The US secretary of state and his Iranian counterpart have held five hours of talks in Switzerland, as the deadline for a framework deal approaches.
Monday’s meeting in Lausanne between John Kerry and Mohammad Javad Zarif is part of a diplomatic push by the two sides to come up with the outlines of an agreement by the end of March.
A full accord is due by July 1.
“If [Iran’s nuclear programme is] peaceful, let’s get it done. And my hope is that in the next days, that will be possible,” Kerry told American CBS television on Sunday.
There were, however, “important gaps”, he said.
Reporting from Lausanne, Al Jazeera’s diplomatic editor James Bays said: “We’re really reaching the crunch moment in all of this. The deadline is supposed to be the end of the month for a framework deal.”
The meeting between Kerry and Zarif included Ernest Moniz, US energy secretary, and Ali Akbar Salehi, Iran’s nuclear chief, who also met on Sunday to negotiate technical details of Iran’s nuclear programme.
“I’m very optimistic,” Salehi said afterwards, leaving Zarif and Kerry in the room for a one-on-one meeting.
Six world powers, the US, Germany, France, Britain, Russia and China, are trying make a deal with Iran that would curb Iran’s most sensitive nuclear activities for at least 10 years in exchange for the gradual easing of some sanctions.
Talks in Brussels
After their meeting in Brussels, the Iranian delegation will return to Lausanne for more talks with the Americans, and will be joined later in the week by European ministers.
Israel and some Western and Gulf Arab countries suspect Iran of ambitions to create an atomic weapon. Iran denies that accusation and says its research is for purely peaceful purposes.
“John Kerry doesn’t just have to do serious diplomacy here to get a deal, but if he does get a deal, he has to do serious damage limitation with key actors in all of this”, said Bays.
“The Saudis say if Iran has nuclear technology then Saudi should have nuclear technology and they signed a memorandum of understanding on that very issue with South Korea just a week ago. Other countries in the Gulf are thinking along similar lines.”
The US and Iran have not had diplomatic relations for 35 years but the 2013 election of President Hassan Rouhani resulted in a minor thaw and a diplomatic push to resolve the more than decade-old nuclear standoff.
Under a November 2013 interim deal with the six world powers, Iran stopped expanding its activities in return for minor sanctions relief.
Since then the parties have been pushing for a lasting accord.