South Korea's military claimed, on Thursday, that a DPRK drone violated the vital no-fly zone around the presidential office last month, during a rare incident it had previously denied.

On December 26, South Korean news agency News1 agency reported that five drones that crossed the Korean military demarcation line "violated" South Korean airspace.

At the time, the drones were described by South Korean Joint Chiefs of Staff as no bigger than two meters in size.

After failing to fire down any of the drones, South Korea's military apologized.

"It is not true that (the North Korean drone) did not pass over Yongsan," Lee Sung-jun, a spokesperson for the Joint Chiefs of Staff said on Thursday, in reference to the area where the presidential office and Defense Ministry are located.

According to a military officer, an inquiry discovered "the trail of an enemy tiny drone" passing past the northern edge of the no-fly zone.

"We make it clear that there is no problem with the safety of the Yongsan office," the official said.

The South Korean military claimed last week that Pyongyang's drone intrusion "could not have collected important information" due to Pyongyang's low technology level.

"It is worrisome that Yongsan, where South Korea's top security control facilities are located, was infiltrated in the height of tension between the two Koreas," Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said as quoted by AFP.

Military tensions on the Korean sea peninsula skyrocketed following South Korea's provocative military drills with the US and Japan. Despite DPRK’s warnings, South Korea, under President Yoon Suk-yeol, has increased joint military drills with the US and is exploring cooperative planning and exercises involving US nuclear assets with Washington.