In this June 21, 2017 file photo, a North Korean drone is displayed at the defense ministry in Seoul after it was discovered in Inje, Gangwon Province, northeastern South Korea. Newsis

A North Korean drone briefly entered a 3.7-kilometer-radius no-fly zone around the office of President Yoon Suk-yeol in Seoul last month, a military official belatedly confirmed Thursday, reversing the defense authorities' announcement that there was no such incident.

The drone was among the five unmanned aerial vehicles that the North sent across the Military Demarcation Line separating the two Koreas on Dec. 26. The South Korean military failed to shoot them down, raising questions over its air defense posture.

"It briefly flew into the northern edge of the zone, but it did not come close to key security facilities," the official told Yonhap News Agency on condition of anonymity, referring to the security area called "P-73."

In a background briefing for reporters later in the day, another military official also said an object, presumed to be an enemy drone, appears to have flown through a part of the "northern tip" of the P-73 zone.