South Korea’s military fired warning shots last week after several North Korean soldiers crossed the military demarcation line separating the two countries, according to South Korea’s Yonhap news agency.

Citing an unnamed source, Yonhap reported Sunday that multiple North Korean troops crossed the Military Demarcation Line, or MDL, in the eastern section of the Demilitarized Zone.

The incident was the first reported violation of the border by North Korean military personnel this year.

South Korean forces responded by firing warning shots, prompting the North Korean soldiers to retreat to their side of the heavily fortified border.

According to Yonhap, the soldiers were believed to have been conducting patrol operations when they crossed the line. No additional unusual military activity was reported after they returned to North Korean territory.

North Korea did not immediately comment on the incident.

Yonhap reported that North Korean troops crossed the MDL 17 times last year, highlighting continuing tensions along one of the world’s most heavily militarized borders.

North and South Korea technically remain at war because the 1950-53 Korean War ended with an armistice rather than a formal peace treaty.