Tehran and the so-called P5+1 group of nations have been locked in talks since February on a permanent nuclear deal aimed at ending a decade-long diplomatic crisis.
A final agreement is aimed at ensuring Tehran will never develop nuclear weapons under the guise of its civilian activities, and would lift international sanctions that have crippled Iran's economy.
"I am confident that a comprehensive agreement is imminently within reach," Zarif wrote in a letter published by his ministry.
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"It requires foresight, political will and recognition of realities by our negotiating partners as well as the audacity to make the right choice benefitting the entire global community," Zarif said.
Tehran agreed in November to curb its nuclear programme in exchange for limited sanctions relief in a preliminary deal which comes into force on January 20.
Zarif said Iran rejected "arbitrary, degrading and unlawful demands" made by other countries over its enrichment of uranium, which Tehran insists is part of a peaceful energy programme.
Iran and the P5+1 nations -- UN Security Council permanent members Britain, China, France, Russia and the United States plus Germany -- failed in November to strike a landmark deal, giving themselves until July 1 to reach a full technical accord.