Amid escalating tensions with the United States, North Korean leader Kim Jong Un tested out a seemingly laughable, but key part of his arsenal.

Footage recently emerged of North Korea paratroopers jumping out of what seems to be two 70-year-old Antonov An-2 transport aircrafts.

While the concept of Pyongyang using these Stalin-era planes seems far-fetched considering the nuclear weapons at its disposal, the aircrafts could be North Korea’s most deadly weapon.

The dated aircraft raisesthe concern that they could be used to transport a nuclear bomb to be dropped quite easily without triggering any radar at any specific target.

According to several reports, the North Korean army has a fleet of 200 Antonov An-2 aircrafts. They were first “introduced” in 2015, but disregarded as obsolete by many.

However, what makes them so dangerous is that they have an incredibly low radar profile, meaning they are difficult to track using conventional aircraft.

The Drive reported the aircraft fly at such a slow speed and low altitude that modern surface-to-air missile systems would have a very hard time detecting and engaging them.

The aircraft, initially designed as a crop duster and utility transport, can land on short sections of a road that would make disembarking soldiers quite easy and sneaky.

According to the BBC, the Antonov An-2 first flew in 1947 as the Soviet Union was rebuilding after the end of World War II. Aside from its remarkable short take-off and landing, it can basically fly backwards.

“The reason the An-2 still flies is that there is really no other aircraft like it,” aviation writer Bernie Leighton, who has flown in an An-2 in Belarus, told the BCC. “If you need an aircraft that can carry 10 soldiers, people or goats, that can take off from anywhere and land anywhere ‒ it is either that or a helicopter.”

The Soviet Union built more than 10,000 airplanes before it fell in 1991.