SEOUL (REUTERS) - South Korea plans to sign a deal to buy 40 fighter jets in the third quarter, its arms procurement agency said on Monday, and its plan to acquire Lockheed Martin Corp F-35s appears on track despite possible development delays.

"With changes made in procurement plans from February to March, followed by committee approval, we will work to sign a contract around the third quarter," Mr Jung Kwang Sun, the head of aircraft programmes at the Defence Acquisition Program Administration (DAPA), told reporters.

South Korea decided to redraw the terms of its 8.3 trillion won (S$9.74 billion) tender to buy 60 fighters last year, reducing the number in December to an initial 40 jets after rebuffing Boeing Co's F-15s.

Although not yet finalised, South Korea has signalled its intention to buy F-35As since its Joint Chiefs endorsed a need in December for "cutting-edge stealth" jets for first delivery in 2018.

"After judging through many channels, we have heard that there is no problem in (the US Air Force) adopting the F-35A by 2016 and we believe there will be no problem by 2018," Mr Jung said.

The remark comes after a Pentagon report seen by Reuters and published on Thursday said a possible 13-month delay in F-35 software development, coupled with maintenance and reliability problems, could stall the US Marine Corps' plans to start using its F-35B fighter jets from mid-2015.

"We're buying F-35As, not (F-35)Bs. Not only that, it's the nature of being in development to encounter problems and solve them," a DAPA official with direct knowledge of the fighter jet procurement process told Reuters.

South Korea's parliament approved some 366.4 billion won for the fighter programme in 2014, in line with South Korea's plan to finalise its choice of jets and make initial payments this year.