Seif was quoted by state television as saying that $28 billion (25.8 billion euros) would go to the central bank and $4 billion "will be transferred to the state treasury as the share of the government".
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The unfreezing of assets comes after the UN atomic watchdog confirmed at the weekend that Iran had complied with measures imposed by the deal with global powers reached in Vienna in July.
The assets can be used "to buy and import goods, as the entry of such an amount of currency in to the country is not logical," Seif said.
The central bank plans to keep the funds "in centralised and safe accounts" abroad, he added.