Couriers carrying millions of dollars worth of gold bullion in their luggage have been flying from Istanbul to Dubai, where the gold is shipped on to Iran, according to industry sources with knowledge of the business.

Turkish trade data confirms the gold is being transported to Dubai by air. According to the data, $1.45 billion of Turkey's total gold exports in August were shipped through the customs office at Ataturk airport's passenger lounge. Almost all of the rest, $800 million, were shipped from Istanbul's smaller Sabiha Gokcen airport.