North Korea on Saturday accused South Korea of a “serious provocation” after Seoul fired warning shots at North Korean soldiers setting up barriers along the tense border.

Ko Jong Chol, vice chief of the North Korean People’s Army General Staff, said the shots on Tuesday coincided with South Korea-U.S. military drills and were a deliberate attempt to escalate tensions. South Korea’s Joint Chiefs of Staff confirmed firing warning shots when North Korean soldiers briefly crossed the military demarcation line in the central border area. The soldiers returned to North Korean territory without incident, and no return fire was reported.

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In recent months, Seoul has used loudspeakers and warning shots to prevent North Korean troops from crossing the border while reinforcing defenses with anti-tank barriers and mines. Ko said North Korean soldiers were carrying out a “barrier project to permanently block the southern border” and had informed U.S. forces in advance to avoid accidental clashes.

Tensions remain high as North Korean leader Kim Jong Un showcases nuclear capabilities and aligns with Russia amid the Ukraine conflict. Kim has dismissed diplomatic overtures from South Korea’s new liberal President Lee Jae Myung, who recently called for restoring a 2018 inter-Korean military agreement to reduce border tensions.