President Yoon Suk-yeol speaks to reporters as he arrives at the presidential office in Seoul on Oct. 14. Yonhap

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.