South Korea plans to finalise the purchase of 40 next-generation F-35 jet fighters from US aerospace giant Lockheed Martin later this year, Seoul's military procurement agency said.

"We will... push to sign the contract in the third quarter... after testing and further negotiations," Jung Kwang-Sun, the Defense Acquisition Program Administration's aircraft department director, told reporters in a briefing late Monday.

The South's air force has been looking to upgrade its ageing fleet of Boeing F-4 and F-5 fighters in the light of North Korea's growing nuclear threat.

Jung did not specifically name the F-35s, but they are the only jet fighters on offer that meet the requirement -- laid down by the South Korean joint chiefs of staff last month -- for a stealth-capable fighter.

The absence of a stealth capability was behind the military's decision in September to block a $7.7 billion deal to buy 60 of Boeing's F-15 fighters.

The Boeing aircraft was the only one to come in under the $7.7 billion budget approved by parliament.

The tender has since been adjusted, although Jung declined to give details on the cost of the 40 F-35s, saying it would require both finance ministry and parliamentary approval.

"If we choose to buy F-35s, we will choose to buy the F-35A that has been adopted by the US Air Force," he said, adding they would be delivered from 2018 onwards.

The F-35A takes off and lands conventionally. The F-35B variant can land vertically, like the obsolete Harrier plane, and its users will include the US Marines and Britain's armed forces.