A senior U.S. State Department official said Kerry expressed to Zarif the importance of negotiating in good faith and abiding to the Joint Plan of Action established in November.
Kerry told Zarif the United States would continue to enforce existing sanctions, Voice of America reported Sunday.
The one-on-one meeting is "incredibly important" in preparation for future comprehensive talks, said European Union foreign policy chief Catherine Ashton.
Another round of negotiations are set to begin Feb. 18 in Vienna.
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