South Korea is unlikely to finalize any orders for the F-35 stealth fighter jet until the summer or fall of 2014.
South Korea's military chiefs last month said Seoul would buy 40 Lockheed F-35 fighters, with the first planes to be delivered in 2018, despite the fact the only Boeing Co's F-15 fighter met the competition's price target.
They say an additional 20 fighters to be acquired could be different models.
Europe's Eurofighter also bid for the order. Both Boeing and Eurofighter have said they remain willing to supply smaller numbers of jets to Seoul, if asked.
The sources said the military chiefs' decision requires Seoul to redo part of its acquisition process and also take another look at the budget, which in turn will defer any move to finalize F-35 orders until the second half of 2014, months later than expected.
Despite the delay, South Korea is not expected to change the decision to buy F-35s.