South Korea is considering purchasing 20 additional radar-evading F-35A fighters to enhance its air combat capabilities, military officials said Monday.

The idea comes in the wake of a nuclear test by North Korea last week.

These fifth-generation jets, which are hard to detect, operate in airspace not accessible by non-stealth planes.

"As North Korea appears to have enhanced its nuclear and missile capabilities in recent years, we are mending our existing wartime strike programs against the North," a military official told Yonhap News Agency.

North Korea has conducted two nuclear tests this year after detonating three other devices since 2006. It has also test-fired missiles, including submarine-launched and intermediate-range ballistic missiles.

In 2013, South Korea paid $7.06 billion for 40 F-35A fighters for delivery between 2018 and 2021. At that time, it had the option of buying 20 additional warplanes depending on the security situation.

Lockheed Martin makes three forms of the F-35 Lightning II. The F-35A is the air force version, which according to its website, "combines advanced low observable stealth technology with fighter speed and agility, fully fused sensor information, network-enabled operations and advanced sustainment." The F-35B is a short takeoff and vertical landing variant for marines and the F-35C is an aircraft carrier-based one.

Last month, Lockheed Martin was awarded a $7 million contract for multispectral database for the planes.

On Monday, the conservative Korea Freedom Federation called for the placement of tactical nuclear weapons in the country. All U.S. naval and land-based tactical nuclear weapons were withdrawn from Korea in 1991.