The South Korean military confirmed Thursday that a North Korean drone had breached the key no-fly zone around the presidential office during a rare incursion last month.

Military tensions on the Korean peninsula rose sharply last year as the North conducted sanctions-busting weapons tests nearly every month, including firing its most advanced intercontinental ballistic missile ever.

Pyongyang then sent five drones across the border into South Korean airspace on Dec. 26, the first such incident in five years, prompting Seoul to scramble jets to respond.

South Korea's military had already apologized after failing to shoot down any of the drones, despite a five-hour operation, which it blamed on the drones being "too small."

It had also repeatedly denied reports the drones had infiltrated the key no-fly zone, known as P-73, that includes the skies over the South Korean presidential office.

"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 Thursday, referring to the area where the presidential office and defense ministry are located.

A military official said an investigation had found that "the trail of a small drone of the enemy" had passed through the northern end of the no-fly zone.

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

'Worrisome'

South Korea's military said last week the North "could not have obtained meaningful information" with its drone incursion because of what it described as Pyongyang's low technology level.

But Seoul's spy agency told lawmakers Thursday "it is possible" that the North Korean drone took photos of the South Korean presidential office, Rep. Youn Kun-young of the main opposition Democratic Party told reporters.

"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, told AFP.

He said the initial denials would also likely damage public trust.

South Korea under President Yoon Suk-yeol has beefed up joint military drills with the United States and is discussing with Washington joint planning and exercises involving U.S. nuclear assets.

North Korean studies scholar Ahn Chan-il told AFP the hawkish Yoon had seemingly become a "feared figure" in Pyongyang.

He said that the drone incursion could have been a test flight in case of a future assassination attempt.