Iran's biggest crude oil tanker operator, NITC, is storing Iranian crude on two very large crude carriers (VLCCs) and expects that number to fall as the winter demand for oil rises, its chairman said.

"At the moment only two ships are now allocated for storage," Mohammad Souri told Reuters Insider TV.

"They're special heavy oil, and therefore not many clients are buying that."

"But they are not going to stay there forever...It (the number of storage tankers) is slowly decreasing. It was seven-eight maybe eight or nine months ago. It is falling with the winter demand," he said.

A shipping source had estimated that NITC was storing crude on around seven VLCCs. A VLCC can store up to 2 million barrels of crude oil.

Traders had told Reuters late in October that the volume of Iranian crude stored at sea had fallen by as much as 4 million barrels because of a spike in Asian demand this month.

Souri said the tanker market was "not too good now" due to an oversupply of vessels resulting from lower-than-expected demand for crude.