The country's air force has been looking to upgrade its ageing fleet of F-4s and F-5s in the light of North Korea's ongoing nuclear threat, with the stealth-capable F-35 widely believed to be the only jet to meet the latest specifications.
The expected USD 7.7 billion deal would see South Korea acquire 40 of the fighters, which are not yet operational, from 2018 onwards.
A South Korean defence ministry official declined to confirm the deal had been awarded to Lockheed Martin, saying the final announcement would only be made next year.
The deal, originally aimed to provide 60 advanced combat fighters, had initially attracted bids from Boeing, Lockheed Martin and the European aerospace consortium EADS.
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Boeing, with its offer of F-15 Silent Eagles, had ended up the sole eligible candidate after proposals from the other two came in over the country's stated budget.
But the state Defense Acquisition Programme Administration apparently found the F-15 did not meet the air force's current requirements.
In late August, 15 South Korean former air force chiefs signed a petition suggesting a review of the "irrational" procedure that weeded out the higher-priced Lockheed Martin F-35 and the EADS Eurofighter Typhoon.
South Korea's military procurement needs, especially where the air force is concerned, have overwhelmingly been met by US suppliers in the past- a reflection of their close military alliance.
South Korea will also reconsider the required operational capabilities and security situations to buy another 20 jets with a goal of deployment starting 2023, apparently giving the two other companies an opportunity to secure a contract.