Hyundai Engineering plans to invest 3.1 billion euros in the second phase of the Kangan oil production and refinery project in southwest Iran, it said in a statement.
The agreement marks one of the biggest investments since a nuclear accord with world powers lifted global sanctions on Iran.
The South Korean firm will have nine months to secure financing for the project -- a potential obstacle given the continued reluctance of international banks to engage with Iran.
"The start and execution of this project relies on 95 percent of the project's financing coming from Korean banks (with) full support from the Hyundai Engineering Company in securing those funds," Arefi told Shana.
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However, a Hyundai Engineering spokesman told AFP that 85 percent of the funding would come from Korean lenders, including Export-Import Bank of Korea and the Korea Trade Insurance Corporation.
Iran has signed initial oil deals with European firms Total and Shell in recent months, potentially worth billions of euros.
But doubts persist over how these deals will be financed so long as Iran remains frozen out of the international finance system by continuing US sanctions.