Shares of both BHP Group (ASX:BHP) and Rio Tinto (ASX:RIO) fell 3% in early trading on Thursday after exports from Guinea's Simandou iron ore project rose in May, according to a media report.

Shipments from the project's Morebaya port were 2.2 million tonnes in May, compared with shipments of 1.3 million tonnes in April, according to ship tracking data from Kpler, the Australian Financial Review (AFR) reported on Thursday.

The project is expected to ship 120 million tons per year once at full capacity.

Traders are waiting to see if supply from Simandou will displace those from Australia and Brazil or merely add to the market.