Correction: The F-35B has a short-takeoff-and-vertical-landing capability, whereas the "A" variant has a conventional-takeoff-and-landing capability.

SEOUL — South Korea will begin the second phase of its plan to acquire stealthy fighter jets, code-named F-X III, by acquiring 20 more F-35s, the country’s arms procurement agency has confirmed.

The Asian economic power had ordered 40 F-35As for Air Force operations under a 2014 deal worth about $6.4 billion, with the delivery of the fifth-generation fighters starting earlier this year.

“The government is preparing to launch the second phase of the F-X III in 2021 for the five years to come,” the Defense Acquisition Program Administration, or DAPA, said in a report to the National Assembly on Oct. 7. About $3.3 billion will go toward buying the additional Lockheed Martin-made aircraft, the report noted.

Which F-35 variant is under consideration has been a point of debate here. Multiple defense sources say the government will buy the F-35A, which is designed to land on traditional runways. But the "B" variant’s short-takeoff-and-vertical-landing capability allows it to take off and land from South Korea’s new large-deck landing ship planned for deployment in the 2030s.

“The state-funded Korea Institute for Defense Analyses, or KIDA, has concluded a study on the additional acquisition of F-35 aircraft, and the study is to suggest the introduction of more F-35As be more feasible,” a source at the Ministry of National Defense told Defense News on the condition of anonymity.

“There are two issues [with getting] the F-35B. First, it’s more expensive than the conventional-takeoff-and-landing version. Second, the deployment of a carrier-type landing ship is far away from now,” the source said.

In July, the South Korean military approved a plan to construct a carrier-type landing platform helicopter ship as part of its long-term force buildup plan. The new vessel is to be refit to displace 30,000 tons, double the capacity of the previous two types with 14,500 tons of displacement.

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On Oct. 1, the Air Force showcased its F-35As for the first time since it received the fighters during an Armed Forces Day ceremony.

The service has so far brought in eight units, with five more arriving here by year’s end. Fourteen more aircraft are scheduled to be delivered to South Korea next year, according to the service.

“For its operational deployment, we are now carrying out related processes such as training pilots and maintenance technicians and the construction of facilities and relevant systems,” the service said in a report submitted to lawmakers on Oct. 10. “As a centerpiece of the country’s strategic targeting scheme against potential enemy forces, the radar-evading warplane is expected to boost operational capabilities and strengthen the readiness posture against threats from all directions.”

The F-35A can fly at a top speed of Mach 1.8 and carry top-of-the-line weapons systems such as the Joint Direct Attack Munition.

North Korea has decried the deployment of F-35 aircraft in South Korea due to the jet’s capability to evade radars and penetrate its territory. In July, Pyongyang threatened to destroy all the F-35As arriving in South Korea.

A senior North Korean official was quoted by the state-run media as saying that the North has “no other choice but to develop and test the special armaments to completely destroy the lethal weapons reinforced in South Korea.”

North Korea test-fired new short-range ballistic missiles and guided rockets in recent months. The weapons take aim at the F-35 base in particular, experts say.