South Korea's military last week fired warning shots after North Korean soldiers violated the military demarcation line, prompting them to return, Yonhap news agency reported on Sunday, citing an unnamed source.
Yonhap said multiple North Korean soldiers crossed the military demarcation line (MDL) in the eastern area of the demilitarised zone (DMZ), in the first reported MDL violation by the North Korean military this year.
The soldiers were likely patrolling the area when they crossed the MDL, and no other unusual activity was reported after the North Korean soldiers returned, Yonhap said.
The North Korean embassy in Singapore did not immediately respond to a request for comment.
North Korean troops violated the MDL 17 times last year, the news agency said.
The two Koreas are still technically at war after their 1950-53 war ended in an armistice rather than a peace treaty.
It comes as North Korea denounced upcoming joint military drills by tens of thousands of US and South Korean forces, warning it could respond with stronger deterrence measures.
The 11-day exercises due to begin on Monday are a regular fixture of the summer calendar, designed to prepare the United States and South Korea to defend against the nuclear-armed North.
