The new tankers, each capable of carrying 2 million barrels of crude, add much-needed capacity to NITC's fleet.
A Beijing-based oil executive with knowledge of these ships said the first tanker with a capacity of 318,000 deadweight tons is named "Safe".
Another seven very large crude carriers (VLCCs) are scheduled for delivery by the end of this year from two Chinese shipyards, and the remaining four are expected to be commissioned by the end of 2013 as part of a total contract worth $1.2 billion, industry executives told Reuters.
A second executive, with direct knowledge of the delivery program, said the Western sanctions have not impacted the shipbuilding contract with Iran as payments from NITC so far have been received without much problem, though the full payment was not due yet.
With a fleet of 39 oil tankers, NITC currently does not have the capacity by itself to ship all of Iran's crude exports of 2.6 million barrels per day, and must either charter more ships or leave it to importers to hire their own vessels.
Running at full capacity, NITC's current fleet has the capacity to ship half of Iran's crude exports to Asia, assuming a 46-day round-trip voyage between Iran and China, according to Reuters calculations.
The shipping firm would roughly need to add an extra 17 supertankers above the 12 it has on order to have the capacity to ship all of Iran's crude to Asia.
NITC ordered the 12 vessels to be built at Waigaoqiao Shipbuilding Co Ltd, a China CSSC Holdings Ltd unit based in financial hub Shanghai, and Dalian Shipbuilding Industry Co. Ltd, based in the northeastern port of Dalian.
Each builder won an order for six vessels, with the total order financed mostly by China's Export and Import Bank, officials said, a policy bank that funds China's massive trade business and Chinese firms' overseas investment.
The Iranian firm has by now settled more than two-thirds of the $600 million bill with at least one of the two shipyards, said the second executive, who declined to be named due to the sensitivity of the matter.