"There is not an agreement at this point," Kerry told reporters shortly after arriving in Geneva today to help seal what is hoped to be a landmark deal with Tehran.
Kerry, who broke off a Middle East tour to join the ongoing negotiations in the Swiss city, stressed that "there are still some very important issues on the table that are unresolved."
"It is important for those to be properly, thoroughly addressed," he said, adding though that the six world powers leading the talks with Tehran were "working hard" to reach an agreement.
The hoped-for agreement - seen as a first step ahead of further talks on a final deal - could see Tehran freeze its nuclear efforts for as long as six months in exchange for some relief from the sanctions that have battered its economy.
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In addition to Kerry, the foreign ministers of France, Britain and Germany also arrived today following last-minute announcements they would join the talks.
Iranian Foreign Minister Mohammad Javad Zarif also cancelled a trip to Rome to continue the talks.
But Kerry, who is set to meet with his French counterpart Laurent Fabius and EU foreign policy chief Catherine Ashton separately today afternoon before a trilateral meeting including Zarif, pointed to continued disagreements and tried to dampen expectations.
"We hope to try to narrow those differences but I don't think anybody should mistake that there are some important gaps that have to be closed," he said.