President Yoon Suk-yeol said Friday that North Korea violated an inter-Korean military tension reduction agreement by firing artillery shots into buffer zones earlier in the day.

Between Thursday and Friday, the North flew about 10 military planes close to the border with the South, conducted artillery firings off the east and west coasts and launched a short-range ballistic missile toward the East Sea.

The actions represented the latest in a series of increasingly threatening provocations by the North.

"We're building a readiness posture against North Korea's provocations without leaving any gaps and by doing our best," Yoon told reporters as he arrived for work, adding that such physical provocations are bound to be followed by psychological offensives aimed at reunifying the peninsula by force.

"For our security, it's more important than anything to have a firm awareness of the enemy and strong commitment to defending the Constitution by upholding our liberal democracy," he said.

Yoon said the North's artillery firing constituted a violation of the 2018 Comprehensive Military Agreement, which calls for halting all hostile military activity between the Koreas.

"We're looking into everything one by one," he said. "But it's correct that it's a violation of the Sept. 19 accord."

Some members of the ruling People Power Party have argued the agreement, signed after a 2018 summit between then President Moon Jae-in and North Korean leader Kim Jong-un, should be scrapped amid the North's increased missile testing in recent weeks.

Last week, Yoon was asked if he is considering annulling the deal in the event North Korea goes ahead with what would be its seventh nuclear test.

"It's a bit difficult to tell you in advance," he said at the time.

Yoon was also asked Friday whether he is open to a preemptive strike on the North in the event it intensifies its provocations.

"What are you talking about? I've already addressed that," he said before countering news reports questioning the effectiveness of South Korea's so-called three-axis deterrence system.

The system, which is built to counter North Korea's nuclear and missile threats, consists of the Kill Chain preemptive strike platform; the Korea Massive Punishment and Retaliation, a program to target an adversary's leadership in a contingency; and the Korea Air and Missile Defense system. (Yonhap)