"With good will we can reach an agreement within six month," he said in a speech to the German Council on Foreign Relations, adding: "I don't fear a decision in the U.S. Congress ... The U.S. president has promised to veto it."
Zarif was visiting Berlin after meeting U.S. Secretary of State John Kerry and other members of the six powers negotiating with Tehran at the Munich Security Conference at the weekend.
Iran reached a landmark preliminary agreement with them in November to halt its most sensitive nuclear operations, winning some relief from sanctions in return. Talks with the six powers about a definitive settlement begin in Vienna on Feb. 18.