Traders said the GTC on Wednesday bought at least one cargo, possibly two, that is either 50,000 tons or 100,000 tons.
"It looks like Iran (GTC) has bought raw sugar for July shipment so it's quite prompt," a trader told Reuters.
"It's unclear the exact quantity, although I would not consider anything above 100,000 tons at least for the moment."
Some European trade sources said commodity house Bunge on Wednesday sold at least 50,000 tons of Brazilian raw sugar to the GTC.
Bunge was not immediately available for comment.
Some traders said the GTC had been making enquiries for raw sugar in recent weeks. One trade source said the GTC had bought more than 250,000 tons of raw sugar over the past two weeks.
Traders said that the GTC had taken advantage of a slide in global raw sugar prices, which hit 20-month lows this week, weighed by expectations of a big global sugar surplus as harvesting in top producer Brazil progressed.